Epic Games Store does support both
modding tools and experimental versions of games, they even have early-access stuff.
GOG also supports branches and at least downloadables/goodies. I already told people this on the M&B modding Discord server, but it's probably a logistical issue for TaleWorlds; they think it's too much work to ship them all at once.
I think it's a good idea to have feature parity for the same price, at least take advantage of the TaleWorlds account system to verify purchases from other stores and let people download files directly from the site (just like the previous trial-ware games) as a band-aid until the game exits early access. Or well, if you have the bandwidth just make the modding tools publicly accessible. They can't be used without the base game.
Ideally we'd have everything everywhere, the company is big enough to automate a release submission, as long as they share files. They should.