That bowing was always the price of admission. To gain access to the debates and to have any chance of pushing the party platform, he had to run as a sincere Democrat, which included a pledge to support whoever the nominee would be. (Frankly, I think there is a strong chance that pushing the party was his whole original reason for running, not because he thought he’d have a real shot at winning.) At the point it was obvious that the game had been rigged against him, he had three options. He could continue fighting fruitlessly from within the party and be blamed for damaging the party, he could welch on his pledge and run as an independent, still have no chance of winning, and risk being blamed as the spoiler if Hillary lost, or he could work to solidify his position within the party and try to grow his faction. The first two options would have handed the DLC bunch a cudgel which they would have wielded to drive out Sanders’ supporters and solidify their stranglehold on the party. Clinton would certainly have attempted a purge of progressives had she won, but by swallowing his pride and stumping energetically for her, Sanders was doing what he had to do to protect progressives within the party from the vindictive fallout of his having dared to inconvenience her by running against her in the first place. But if Sanders campaigned energetically for her and she still managed to lose, that would present a historic opportunity to shift the base away from the DLC wing and towards the progressive wing. The polls that showed he would have beaten Donald were the icing on the cake, because that demolished the “electability” argument which has always been the main pillar of DLC support.
So Sanders lost cred with Greens and outsiders who had no chance of threatening the Duopoly from the outside anyway, and in exchange, he became one of the most respected, credible, and popular leaders within the Dem party with a real shot at transforming it from within. That’s not a bad trade. Even if by some miracle, he had had some chance to become president last time, he would have been an isolated man facing two opposition corporatist parties, and they would have just kept him tied up until they could sweep back in and erase any trace that he’d ever been there as quickly as possible. If Sanders succeeds in helping the rank and file take over the Dem party, the lasting impact from that could be far greater and more enduring than anything he might have accomplished as President.
To me, it looks like Sanders has been playing the long game brilliantly. Unfortunately, he probably doesn’t have enough time remaining to benefit from that himself, but I don’t think in his mind this has ever been about him. I think he’s thinking in terms of what he can leave as his legacy. I expect he would love to have the help of his former supporters, but if some of them are so butthurt and disgruntled that they can’t do anything but throw rocks at him and the Dems, it would not surprise me if he uses that to help solidify his position within the party as well.