As far as the concept of 'niche protection' is concerned... the way that D&D has currently set themselves is that most characters can have the opportunity to do anything, but that some classes/characters are more focused on it and have a much easier time with it. Which is I think the best way to go-- if you want something done very, very well... having a PC of a certain class will accomplish it. But if no one ends up playing that class, you can still get the job taken care of by someone else. They won't be the best possible player at it (some might uncharitably say ;overpowered' at it), but they can still get the job done as needed.
That's why any PC can have the opportunity to gain proficiency in Thieves' Tools to open locks and disarm traps (even though Rogues have additional abilities that make them better at it)... and why any PC can have the opportunity to have healing magic (even though Clerics and the Life Cleric specifically have additional abilities that make them better at it.) A party can cover almost all the standard adventuring party bases regardless of the classes people choose, just with the right selection of backgrounds, subclasses, feats and so forth. Each class still have a 'niche' of what part of the standard adventuring bases they are best suited for and can absolutely be the go-to for the party when it comes up... but they aren't required to be there.
I do not foresee a time where WotC returns D&D to the design space where only one class can cover a very important adventuring base and if you don't have that class in the party then that base cannot be covered in any way, shape, or form. That is an unnecessary protection of a so-called 'niche' that would ruin the distribution of class selection and no gain the game anything worthwhile.