I'm laughing so hard that tears are rolling out of my mouth-tentacles. This is the same weak argument you've used throughout the thread.
Give me a single reason why we shouldn't have a Psion that isn't "it's unnecessary".
I'll be waiting.
Here's 7.
1) It makes multi-class combinations which are unbalanced more difficult to predict as the number of classes (as opposed to sub-classes) increase;
2) Any themes it contains which overlap with existing themes from other classes will dilute those other classes;
3) It establishes psionics as equal importance to magic (after campaigns have run for sometimes 6 years with little psionics) which can upend setting themes where psionics are not of equal importance to magic - making them subclasses allows a lot more control over how prominent they are as their own separate "thing" in the setting because it eases in through only a few level touchstones, but a separate class would establish it across all 20 levels and be much more noticeable and therefore more prone to breaking the verisimilitude of established campaigns. And while yes a DM can always control their setting, it's more complicated when you use rotating DMs where one likes Psionics more than the other, or where a DM wants his players to enjoy the use of new books they buy and tries hard to incorporate it but has more difficulty absorbing a full class on a new theme as opposed to a subclass.
4) It increases the odds of option paralysis, where the number of choices presented by a subclass is significantly fewer than the increasing the number of classes and therefore it's own subclasses.
5) It increases the odds you will have a problem of the haves vs the have-nots in games. When new subclasses are introduced this can be a minor issue, but they're still just new sub-themes to existing classes which are typically viewed as not game-altering in nature as "the new hotness" that players who cannot afford that book might want. However a new class, with it's own sub-classes, risks it being more of a major issue where the prominence of the theme can be viewed as "the new hotness" and therefore cause a sense of inequality in the game when some players might want to enjoy that material but cannot access it / afford it (and while you might suggest they can just borrow it, that has it's own equality dynamics - people cannot afford something don't like admitting they cannot afford it, and might feel embarrassed asking to borrow it, and might feel compelled to spend money on it they don't have because of those feelings).
6) Psion as it's own class isn't as popular, according to the Data WOTC has access to and you do not, than using them as a sub-class. Giving more people what they want is a good thing.
7) I don't want a full-class Psion (anymore) and spending limited time and resources working on something I don't want them to work on means they cannot spend that limited time and resources working on something I do want them to work on.