I'm not sure, if you want to read another covenant thread or not. I just want to explain several things to players, who don't understand, why covenants are implemented and therefore they're against this change.
First of all, I was in your shoes. I also was against some doubtful changes in a past, such as overtuned content in Cata, mandatory dailies in MOP, removal of flying in WOD. But I'm on the other side of barricades this time.
Blizzard don't want to tell it directly. But they have been having major problems with alts since WOD. Casual/alt-friendly content of WOD allowed players to level many alts. And Blizzard didn't want to support such playstyle. May be because they don't want to inflate their server DB size. Yeah, we pay sub fee for 50 character slots. But it wouldn't be profitable for Blizzard, if all of them would be used.
But most likely they didn't want to support alts, because THEY DIDN'T WANT TO DESIGN THEIR GAME AROUND EVERY PLAYER HAVING EVERY CLASS AND SPEC.
I think, I don't need to explain, why it's bad. Because when player has access to every class/spec, he is able to be more optimal for every encounter. And therefore encounter should be designed with this fact in mind. And this would make it harder for player, who don't have all classes/specs available. So it would literally force him to have all classes/specs. And whole class/spec system should be made with this in mind then.
That's, why they tried to restrict alts in Legion and BFA. Large amount of per-character grind/time-gating/RNG was intended to restrict your ability to play alts. Especially in BFA. But BFA failed exactly due to this reason.
And now they seem to give up. In Shadowlands you will be able to level new character within just 2 hours. While it's not "instant change class" ability like in Firefall, you were asking for, but it's very close to it.
So, now Blizzard seem to be ready to start designing game around every player having every class and spec. It's solution of another problem - problem with class balance. We have so many classes (12 or 13? I've lost count), around 40 specs and it's possible that we'll have even more in a future. It becomes impossible to balance all of them. That's, why we don't have new ones for the 2nd xpack in a row.
So Blizzard just try TO REPLACE CLASS/SPEC SYSTEM WITH COVENANT SYSTEM. I.e. replace over 9k classes with 100500 specs with just 4 covenants. So, that's why covenants have to be "permanent choice" system. They're actually replacement for classes. Plus borrowed power system. Blizzard will be able to make new interesting "class" system in every xpack.
You should just understand it. It's not bad change. Ion just isn't so good CM, as GC was. He just don't want to explain their real intentions. Players don't understand them and therefore they're negative about them.