Feels somewhat similar to the faith of RethinkDB, which was an awesome database in many regards, but it was difficult to keep the steam up after the sponsoring company shut down.
https://github.com/rethinkdb/rethinkdb/graphs/contributors
One thing that may work, is if Servo tries to carve out a very narrow niche, much smaller than just "general browser", where they are unique and useful.
Take embedding as an example. If Servo became a great choice for embedding (similar to WebViews, Electron or maybe as an "engine" to Electron), where there is usually a single set of CSS and Javascript that it's expected to run (basically a bundled app), the downsides of Servo not supporting all Web APIs, or not handling all edge cases, is reduced. Basically, if it runs your code, that's enough.
So it'll become "useful" (regardless of where you put the bar for useful) earlier in an embedding context, compared to it becoming useful as a general browser.
Should it catch on as a tool for embedding, that would drive more usage and more contributions, which would be a beneficial circle.
But this niche could also be something entirely different, of course, I'm not sure embedding is the one.