Mandatory forms make sense when they affect your gameplay. When you go Feral you stop casting spells and start fight with claws and bites so shifting into a form is impactful for Druids. You can drop form to cast spells and it creates a gameplay choice which is neat.
Shadow Form doesn’t do that. You’re still a caster with/without Shadowform up and it doesn’t even prevent you from casting Holy spells like it used to.
The only gameplay effects it has are a small damage increase and access to your Insanity spenders, but given that there’s no interesting choice to be in/not in Shadowform for a shadow priest (ie you lose a lot and gain nothing by shifting out of it), those bonuses should be baked into the spec and Shadow Form should be made a cosmetic spell for people who like it.
Don’t get me wrong - I do like it. Usually. But I also like seeing my character, seeing my transmog, seeing all the cool gear and looks that I collect. The visual effect can be tiring to see day in and day out as someone who has an almost chronic addiction to transmogrification and toys. The smokey clouds can also interfere with other visual effects on armor, toys, and spells. It’s the primary reason that, despite playing a Shadow Priest a lot since I joined the game in Wrath, the class has never stuck with me for long and I always end up migrating to other classes where the mechanics don’t impact visual appearance so much.
This was the argument made for the removal of perma-Tree of Life form back in Cata or whenever that was removed: it didn’t fundamentally change Resto’s gameplay compared to caster form (it just made them a slightly better healer) and I think the same logic applies here.