With subclasses, prestige classes become redundant

Of course not. Prestige classes don't do the same thing that subclasses do. They aren't a specialisation of the abilities within a class, they're a broadening to a set of features that could theoretically be taken by characters of any class, but which is not in depth enough to justify being its own full class.

But talking about things in the abstract is pointless. I'd rather point to actual concrete examples. Because the 5e homebrew community has shown categorically that there is a useful place for Prestige Classes. This vampire prestige class is the one that most convinced me of it, but you can find a whole heap of good examples of prestige classes in the homebrew community. Sure, you could design a full 20 level vampire class (and 4th edition did!), but it really works better if you have a highly flavourful and more succinct series of abilities. Get in, give you the abilities that satisfy the fantasy, and finish there. But to make vampirism a subclass doesn't make sense. Which class would it be? Rogue? What if you want to be a wizard who gets bitten by a vampire? Do you now have to multiclass into rogue, and only get vampire features once you reach the 3rd level of the multiclass? And not until the 9th level before you get more vampire stuff? That's just this one vampire prestige class, but any time you have a concept where that same reasoning could apply, that's an example of a concept for which the prestige class makes sense.