This looks great, although on first glance seems that the new GL implementation is a new library, not an extension of Leaflet.

I think that's a mistake — Leaflet is solid and is the foundation on which a lot of extensions are built, to the point where my impression is that it's pretty much the de facto JS-based map widget these days.

While the new library looks decent, I don't see the point of creating a brand new library rather than extending Leaflet, other than to force clients to couple tightly to Mapbox's API, tile server etc.