But why do we have a Monk when you could easily play a Fighter and have your DM say that the weapons are actually your fists and feet?
The argument is not why; it's why not? I understand the concern that class bloat will accomplish the negative things that it did in other editions (yes, I know they just introduced the Artificer), but is WotC really so afraid of introducing a new class?
And furthermore, what is your argument against the introduction of a Psion? Saying we "don't need one" is meaningless, because that can be said of any class. Do you need a Sorcerer? No, why don't you just play an Evoker Wizard and user your imagination to pretend that it's a Sorcerer?
This is why I view these arguments with such skepticism. Sure, there's no cabal against the Psion, but there are people whose only argument against it is that it's unnecessary. In that case, then why do we make new subclasses, like the Samurai, when you could just play a Samurai using the Battlemaster subclass?
What is even the argument of people saying this? It's so meaningless as a contention.