WebGL is notoriously hard to use and requires tons of code for even simple things like drawing lines and polygons. (Disclaimer: I'm also one of the core developers of Mapbox GL JS)

One of our core devs (Ivan) works on Leaflet WebGL renderer, but I'd like it to remain a plugin, so that Leaflet stays true to its goal — to be as small and simple as possible.

Indeed! From my point of view, WebGL rendering for Leaflet makes sense, but it's very important to not remove support for browsers which don't implement WebGL right now.

MapboxGL is very performant, but it changes all the abstractions to the map elements, which is something I'm particularly not very happy with. Thus Leaflet.GL.