Are there any arguments against keeping "copies" of WebGL 2 method as WebGL 1 extension method?
The main arguments against are:
The functionality will eventually arrive anyway with WebGL2
Because 1. and resources are tight so lets concentrate on 1.
WebGL2 will not arrive shortly to everywhere. As the year draws slowly to a close, it becomes less likely we'll see any publicly released WebGL2 implementation. WebGL2 implementations will arrive slowly, and updates to UAs that implement them will appear slower on some platforms than on others (Android, Edge and iOS will take forever).
It is my estimate that it will take until 2020 before we'll see an activation rate > 80% for WebGL2.