It would have been nice for Mozilla to stick with it, but on the other hand, my understanding is that Servo originally was more of a proving ground for feature

It would have been nice for Mozilla to stick with it, but on the other hand, my understanding is that Servo originally was more of a proving ground for feature that would incrementally get ported over to Firefox itself (some of which did land, like webrender and I think the CSS processing). It's disappointing that Mozilla wasn't more interested in investing in their long-term browser improvements, but given the level of concern that seems to be growing in recent years around their direction, maybe in the long run having a new browser come along and utilize it would end up being better for everyone. If we're lucky, maybe in a few decades we'll be having debates about whether to call something "Servo/NewFancyBrowser" or just "NewFancyBrowser" as a fresher alternative to the whole "GNU/Linux" versus "Linux" thing.