If I adopt this approach, the new spells that are classified as psionics would not be freely available to other classes as a general rule.
When a "regular" bard say wanted one of those, we would discuss that fact that they are, at that time, gaining a psionic ability. (with rp and campaign considerations attached)
Those spells don't have a tag that says psionic. I fully endorse adding one if a person wants to make them more significant and I suspect many people would do just that.
It's definitely an option to keep psionics separate but it looks to me like the other spells listed as appropriate to a psionic character would need to have some similar consideration. There are over a hundred spells and only ten new ones.
Are you thinking only the new spells would be true psionics? That limits psionic ability but it's a possibility. A person could simply call that the psionic power list and consider a power and a spell version. That seems a bit awkward to me if a person mix and matches but powers them with the same spell slots anyway.
Saying it should be only a subclass is like saying warlock, bard, ranger or paladin should be only subclasses.
I disagree. I think psionicists can be a full class but I'm not seeing how adding a pseudo-caster is necessary. I would like to see a full class but if all it's going to be is another flavor of what we already have when WotC is telling us it's not an option they expect to see utilized then there's no real point to it.
I can already give sorcerers spell points and handwave components then give it a different label. Those other classes were included based on feedback during 5e playtesting. A full psionicist class will be created based on market research and feedback (in theory).
The player base would need to demonstrate the demand.
At some point you have to branch out or you are going to run out of design space.
You mean like monopoly? Coke? Maybe create a new system, see it's not that popular, revert back to the original recipe, and see a huge success? The New Coke model.
That's just a bit of kidding and I understand your point, but design space is finite regardless. At some point there will always be that limit where more or new stops being productive. Branching out in a direction after doing an analysis to determine there's no market in that direction is not productive. Giving up resources on a smaller scale for existing fans is fan service, which is where it looks like we're at when it comes to psionics.
High levels have historically been little-used in D&D, too. But 5e goes to 20th, just like 2e & 3e. The Bard wasn't exactly popular in any prior edition (was consistently mocked for a long time, for that matter), nor the Druid, they both made the PH1 cut.
Being mocked isn't synonymous with unpopular. We would need data from WotC and their reasons for justifying that inclusion to make the popularity comparison without jumping to conclusions.
5E is exception-based so surely that wouldn't apply? The language is quite explicit. Adding Sneak Attack to Rend Mind seems just obscene.
Sneak attack doesn't apply to
rend mind because
rend mind is not a weapon attack. What's more interesting about
rend mind is that the DC formula isn't standard. It's 10 + proficiency + INT mod instead of 8 + proficiency + INT mod. I don't know if that's an intentional rule breaker or if it's a typo but it looks like it could be significant.
Keep in mind
rend mind is at the same level we see
thief's reflexes and
death attack. It's not like rogues don't have options to pump out some high damage at that level.