I think making Mono and the existing .NET Core runtime 'runtime options' simplifies the confusion a lot. Unifying the standard library was already a good idea, and had been partially done anyway. The existing Mono version of the stdlib was a good effort that historically hasn't stacked up when you try to port stuff (and incrementally had been replaced with chunks of the 'you can look at it i guess and it's sort of like desktop netframework' reference sources).

I'm personally happy to see more of the .NET Core stuff end up in Mono since it usually is battle tested and has better compatibility, and I'm also happy to see more stuff approximating the existing .NET Framework and Mono workflows since .NET Core has historically been unable to do important things that netframework and mono can do.

bias disclosure: I work on Mono