I think you're looking at it from a different angle than it's intended. Instead of considering it a change of class, consider it more like retconning the class. We're changing who they've always been. Sure, it's immersion breaking, but it's also immersion breaking when I get up to refill my glass.
The option being offered is "hey, so you think Shadowheart is more interesting as monk or druid? Knock yourself out!" It's just a different way to tell the story
I mean, I see your point but you can already make Shadowheart a druid or monk via multiclassing. It's bad enough that you can also make her any other class just as easily and I would personally leave unrestricted multiclassing and removing that 1 cleric level to mods. Before someone asks 'but what's the difference if a mod does it and if it's a built-in feature?', I'll try to answer that too: It's a way for Larian to set respectable boundaries. That way they say "This is Shadowheart, she's a devout cleric of Shar. This is our vision for her and if you choose to deviate from it somehow, the game may respond in incoherent and immersion-breaking ways." This is a message I'd respect. Class respeccing and the removal of multiclassing requirements is more like saying "This is Shadowheart, she's a devout cleric of Shar but you know what, who cares, make her a barbarian or wizard if you want lol". I can understand that to some this is simply Larian being inclusive and wanting everyone to have fun, but to me this is them being a DM who can't say no even to suggestions that break the lore of their world.
I can't speak for everyone, but I know that when I open Kingmaker and ask "hey, Valerie has great stats for a Paladin, can I make her one?" and the game firmly says "NO", I have much more respect for it and its creative vision than for a game that lets me turn a githyanki soldier into a bard or remove all druid levels from a literal archdruid.
I'd like to say though that viewing it as a retcon didn't occur to me and it helps me see a perspective from which this is not so horrible. I liked how Deadfire handled this - each companion had a set of 3 lore-friendly class and multiclass options to pick from. This was a fine compromise that allowed some variety while keeping everyone restricted to options that made thematic sense for them.
So if Larian put a big fat warning up whenever you want to respec a companion away from their default class, letting the player know that they're breaking this particular character, then it's all good?
