An approach specifically tuned for the Rambam's opinion is found in the Kessef Mishna in hilchos Mamrim at the beginning of chapter two. He asks if a Beis Din which bigger in number or wisdom can argue on a previous Beis Din, why don't we find Amoraim arguing on Tanaim? He answers "perhaps since the sealing of the Mishna, there was a general acceptance (kimu vikiblu) that the later generations would not argue on the previous ones. And this was done as well at the sealing of the Talmud, from the day it was sealed, no-one had permission to argue on it."
Along this line of logic, we do find terms like Kim Li, and concerning a judge Mikabel Alai, which completely bind a person to a given ruling. The very fact that we accepted the Talmud as binding, is exactly what makes it binding.
From here on is a related discussion, but not limited to Rambam's view.
The Rosh in Sanhedrin chapter four #6 quoting the Baal HaMeor and Raavad who discuss arguing on Geonim, points to the gemara in Bava Basra 130b (#פ in Pilpulei Charifta) which states we cannot rule based on a teaching or a practice, until there is a ruling 'halacha limaaseh'. The Rosh seems to be saying that the later Amoraim's ruling for practice is the limaaseh based on the earlier teaching of the Shas. That is what makes it a ruling for us. But seemingly concerning any halacha in and of itself without a practical ruling, the Talmud itself says it should not be relied upon. So it is not so much that we are bound to Shas, but we are rather using the rulings of later Amoraim who used the shaas to rule. That is the binding factor from which we cannot deviate as we have nothing else to rely on. He leaves the idea possible that later generations could in fact argue on Geonim as long as they have a real proof from Shas. But while stating all this he takes for granted that no-one can argue on the explicit ideas that Ravina and Rav Ashi set up (סדר) in Shas.
This idea that the very fact that Ravina and Rav Ashi 'set up' Shas is what makes it binding might be understood by another couple of sources saying we cannot argue on the end halacha of Shas.
This would be the way the Or Zarua siman 753 understood the gemara in Bava Metzia 86a which states 'Ravina and Rav Ashi sof horaah'. He said "we should not rely on any braisa or the Talmud Yerushalmi if it disagrees with the Bavli because the Bavli was written in order to instruct practice (lihoros) to the Jewish people , this is the meaning of Ravina and Rav Ashi were the end of Horaah. However it's always good to bring a proof or better understanding from those sources to the Bavli". (Note this is not like Rashi's pshat there). According to him, our gemara told us we were bound to it and it was written accordingly in a way to be binding.
The Riff at the end of Eiruvin says a similar point as the Or Zarua that only the Bavli and not the Yerushalmi is binding. But his reasoning is different, more in tune with the Rosh. He says that "the Bavli was finished later and the Babylonian Rabbis were more learned in the Jerusalem Talmud than us" (I assume by 'us' he means Jews on the western front for whom he was writing, who might assume they are not bound to the Bavli because they have the Yerushalmi, this he says is not true for the aforementioned reason).
Again, like in the Rosh, we find this idea of hilchisa kibasra. It is a matter of logistics. We need to rely on our Rabbis and the later ones were the most informed.