You may be right -- there's no way to really know, so this is all just navel-gazing, really. But that was almost certainly the calculation that the Sanders campaign made. It's hard for me to get too worked up over politicians keeping their word, though, even if I would have preferred a different end result. 

It's a rare enough thing to celebrate, and demonstrating that your word can be relied on makes it much easier to make progress with the entrenched political powers. Changing an entrenched system requires people working both on the inside and outside of that system. Sanders has always been one to work from the inside. You are taking the perspective of working from the outside -- which is also valid and of incalculable value.