Beginning with a clean slate is always easier. ES 3.1 is so different, it affords this opportunity. There is no backwards compatibility chain for existing developers. Everyone is starting again with a new API.
I'll prefer WebGL 2.0 because it's quite likely it'll arrive next year (around 8 months) and it'll support a lot of desktop and mobile hardware. WebGL 2.1 would probably not arrive next year and its support for desktop and mobile hardware would be substantially lower than WebGL 2.0.
MANGLE is the hell you're speaking of.
A Web technology that doesn't work for most people is not very attractive.