Hatmatter
Laws of Mordenkainen, Elminster, & Fistandantilus
Perhaps it is gone. But, I recall Jeremy Crawford saying a few times in DragonTalk interviews that no idea is gone and that the Wizards of the Coast designers love to go back to ideas that were not embraced and revise them until the team finds the correct iteration that is embraced. The Rune Scribe becoming the Rune Knight is but one of many examples. So, we very well may see another attempt at the Truenamer.So when WOTC tried to jam truenamer in the 5e wizard, the fans rejected it. Not because they didn't want the truenamer, they didn't want it in the wizard. So that idea is now gone.
It is for this reason that I expect, at some point down the road, perhaps in a few years, we will get a Psionicist class. Until that point, I think the subclasses of the Psi Warrior, Soulknife, and Abberant Sorcerer are great ways to manifest psionics in a way where people can use them form day #1 with minimal impingement due to having to learn a new system.
Now, that being said, I realize that was not your main point, so I do not want to represent it as if it was. You are, if I understand you correctly, objecting to how the emphasis on subclass design is impinging on creativity and expansion of the game because the subclasses are too connected to the classes. Point taken.