Larian is hand-cuffing certain aspects of the game for Larian reasons.
Possibly for it to balance the extra things they want to add.

I do not believe a true 5e game couldn't be more faithfully adapted.
I just think "that game" isn't what Larian believes will be fun.

I am placing a lot of faith in the mod community to give us what Larian won't.
Like heavy armor proficiency for my Light domain cleric.

My fear is they butcher multiclassing like the reaction system.
And modders can't fix it.