While I can conceive the value of Shepard's sacrifice of his/her own life and the expiation through it for his/her choices that caused the sacrifices of others, I disagree completely on the "Entropy" theme: here we come to the opposition of Nihilism to Utilitarianism. Here below I provide very quick analysis of what I mean...
You talked about "Entropy", but then what about "Hope", "Free Will", "Cooperation»?
Concerning the "cooperation" Javik confirms that it is a new quality when describing his cycle: his people dominated the galaxy, the people of Shepard's cycle cooperated, this should have changed the output of the things... unless the NIHILISM is the only value that game wants to transmit to its public.
Now I will explain you why I consider such vision (promoting Nihilism) as morally extremely unacceptable from Bioware artistic team, or otherwise speaking, a bad thing to do.
We will go back to a real life: under the "entropy" hypothesis, whatever we do, it does not matter as we will go back to repeat the same cycle again and again and breaking the cycle necessarily means the total destruction, a chaos (destruction ending)
The control and synthesis endings are kind of compromise made with the quintessence of evil - both are like signing the treaty with a devil and I am sorry ... it is not a real life! It is as if we had made a ceasefire and a peace with German Nazis in the Second World War instead of marching to Berlin, waging a full scale brutal war, destroying their armies and industrial power definitively and finally judging and executing the most important Nazis that commited the human crimes. It is not as if USA had not thrown two nuclear bombs on Japan Empire to cut down the war there as well! In the same way, as you say that Sacrifice is a central theme of Mass Effect series, the Reaper's character as an absolute menace for everything that is good, alive and intelligent is well established: no compromise is possible without compromising the ethical values of the galaxy races: The series is constructed in such a way that either Reapers disappear or the galactic civilisations disappear, the other choices are intellectually very unsatisfying : they imply that in all extreme situations the bad compromises should be made: no coexistence is possible between galactic races and Reapers. The "control" ending is nothing more than a bad "cease fire" buying some more time to the surviving races and the "synthesis" does not seem to make any sense from "scientific" point of view and seems like a "peace treaty" with Reapers, which is ridiculous (makes me think about those cases where Afghan women are forced to marry the guy that raped them, the criminal and victim are forced to live in peace: we know that THAT DOES NEVER WORK - that denies any concept of elementary justice). All in all, it brings us 3 bad endings, confirming the hopelessness - the future does not matter: it will be always the same; Nihilism rules the Mass Effect world in this interpretation.
So, you forget about the whole "Hope" theme present in ME series: in two and 90% of the third game Shepard led the people forward and he brought them a hope, that things can be different - it is how the biggest civilisation advances were actually done! It is a very human feeling. Shepard may win in my opinion, not every war has been won by the sheer military force, I like to come back to previously mentioned Second World War (which is transpiring everywhere in Mass Effect): D-Day operation in ... Normandy was prepared under full cover and with a lot of disinformation in such a way that Germans were not aware of the whole thing until it was too late, yet they had a crushing military advantage! The whole occupied Europe was full of resistance that committed the acts of sabotage even though the enemy responded with organized terror on the civil population. And if you looked on the history in other human civilisations, you can find thousands of examples that confirm that thesis... no oppression is eternal; the empires constructed on the violence always fall faster than the countries constructed on the mutual comprehension.
And even a sheer military force is always an option to win if some military expresses the command genius : In the darkest hours of 1941-42, the Russians had conjured the whole human strength they were able to pull to stop Germans some 50 km before Moscow and general Zukow 's command managed to stop the catastrophe! Ok, you say, Reapers are far beyond anything that exists in galaxy ... but what about strategy and tactics? Small real life example: Japanese resisted all tentative of European or American colonisation in XIX century, even though they were technologically weaker: they managed to adopt and defend themselves.
In real life the sacrifice serve for something: The Sankt Petersburg (Leningrad) inhabitants starved and had eaten their shoes, but they did not let the enemy in! Jews in Warsaw and then Polish made an insurrection in front of the forces highly superior to their own and they died, while they were slaughtered, "reaped" ... the whole damned city of more than 1 million people had been destroyed and citizens were killed, yet they held the damned line to the end. There was always a hope. Coming back to Mass Effect, even Edi, in her humanization efforts, mentioned something similar in dialogues with Shepard: She wandered why people prefer to die than preserve themselves by all costs; the compromise with ultimate evil is not preferable to death. Bioware's game endings contradict that HISTORICAL truth.
The fact that Shepard is given 3 solipsistic choices in the end out of nothing denies any sense of the sacrifices done previously by game characters, nations, races.... It goes back to your sacrifice theme and in my opinion counters the whole "Entropy" thing. The entropy "thing" is a game breaker and morally speaking unacceptable vision of the world.
Then about "Free Will": Mass Effect repeated again and again that it is all about the choice ... but Shepard is silent in the end! Where is his free will exposed in there, why doesn't he question the logic of the Catalyst, why cannot he say "NO" for all proposed solutions? In real life the "No" option always exist, "the third way", the option, not foreseen by the one that fixes the rules, "NO" defies the very concept of Nihilism. Shepard was able to say "No" in all of earlier games ... why it is not possible in ME3 ?!
So when Bioware pulls out the ending, where Shepard basically goes over it, I find it personally insulting (coming from Poland, the country that managed to overthrow more evil regimes that I could possibly count) and most of people knowing the history would have a similar feelings about the thing.
One would say that the "war assets" number is important, but ... frankly, in most of human activities, the numbers are insignificant, and the qualitative, not quantitative, values are the most important: none of that is addressed by the endings choice nor their result. Who cares that war assets were on 2700 or 3500? What is important is elsewhere: did we make a peace between Quarians and Geth? Did we save Rachnis, did we manage to close the breach between Krogan and Turians? I repeat again to all pro-enders: the War Assets number is meaningless in the role-playing terms, what is important is how qualitatively speaking we had changed the world.
So what is my point?
Entropy, Nihilism is a very negative approach to the world comprehension: Bioware, opting to construct the ending in such a way, contradicts the very majority of humanity that wants to believe in a better "tomorrow". It contradicts also the themes of hope, cooperation and free will, that in my opinion, are as strong as the sacrifice theme you mentioned.