He did promise he would, so that helps to explain some of the anger and disappointment with his endorsement.
But it's worth considering all of the strategic advantages of endorsing Hillary now, getting the disappointment over with, so that people will have time to get angry and get engaged with the next phase of the movement's goals (same as before, sans the presidency).
There are some drawbacks to waiting till the convention, having the vote and losing anyway. Peoples' emotions would be all focused on their anger and disappointments, when folks in Philly need to be focused on the fight and winning the battles they can. Letting go of Sanders' presidential hopes now will help with that.
As it stands, he still hasn't conceded and released his delegates. They will be going to the convention to vote for him, same as before. So in that respect he hasn't broken his promise to take it to the convention.
But considering he doesn't have the votes to win, and he very likely would have lost with or without endorsing beforehand, there were advantages to endorsing now. See this for a few. See this for a few more (as well as why he's not going third party).
Finally, Sanders' next steps, creating organizations to get hundreds of progressives elected (including independents, not just Dems), is exactly the kind of democratic phase II I was hoping to see from Sanders' efforts.
At the end of the day, I believe Sanders is more than doing his fair share of the work to change this country. It's time to recognize that the real work is up to all of us. Going into Philly and beyond, the baton has now been passed to us to lead.
See this for more on the need for DIY activism post-Bernie.