I don't even understand why "niche protection" is a thing. Could somebody explain to me why it's desirable?
I'll try.
The whole point, i think, of a class-based system with adventuring groups or parties is that it somewhat encourages or even gently forces some inter-party dependence: each character has serious weaknesses that can only really be offset by other characters having strengths in those areas, while you help cover off for their weaknesses.
Fighter's can't heal or cast spells or sneak. But they can
fight.
Mages can't fight or heal or sneak. But they can
cast spells.
Clerics can't sneak. But they can
heal, and do a bit of the other things.
Thieves can't fight or heal or cast spells. But they can
sneak.
The bolded are, of course, the clear niches those classes (or class groups) occupy. Each has weaknesses the others can fill in for, an each has something it's really good at that the others can't do. Sometimes it's even a good thing to have some built-in redundancy by doubling or even tripling up on one or more of those class groups, such that if-when you lose someone you've still got a backup to fill that gap.
Once you start eroding those niches to the point anywhere near "any class can fight
and heal
and cast spells
and sneak" then you have no further need of the rest of the adventuring group: you can do it all on your own as a one-man band. And at that point, what's the point of designing the game around the 'adventuring party' paradigm?
The problems arise when you get to having so many classes that there's just not enough niches or even sub-niches to go around. A few other classic niches past and present have been
woodscraft (Rangers),
dealing with the dead (Necromancers),
mind-messing (Illusionists and-or Psionicists), and
music/sound (Bards).
There's also some things that everyone should be able to do, or at least try, and thus aren't good to use as class niches. Tactician isn't a niche; everyone can think and make plans. Talk-droid isn't a niche; everyone can talk and (try to) be diplomatic. Explorer by itself isn't really a niche; everyone can, in their own way, explore.