If you're asking for my honest opinion, I'd say "we'll have to wait and see." MathML has its share of difficulties, but the biggest is that it is designed as a presentation markup language. That's great for the e-textbook use case, where authors would presumably write up formulas and proofs in some other system (and by that I mean another application, e.g., Mathematica) and have it converted, but editing MathML in GUI form remains a largely unsolved (or unoptimized, at any rate) problem. FireMath is decent enough, but can you imagine trying to integrate something like it into Sigil? That would be a lot of work.