No, I fully get that. What I don't get why people keep wasting time for clamouring some perfect pipe-dream vision of a Psion with completely new bespoke mechanics that WotC is never going to print. If you have such a clear vision, write it an use it in your own games. Problem solved.
We aren't
The topic asks what's the big deal. So we answered and gave our wishlists. Very few people expect to get much if any of their Psionics wishlist from WOTC and few stated so.
No it won't. At least not more than the current multiclassing. (Which is a problem and should be addressed, but also optional.)
Yeah it will if the optional rules or multiclassing and feats are used. And WOTC half expects you to use them. The most broken element of multicassing is full casting as it almost fully stacks and their tier features are tied to the spell system and not the classes themselves.
The true brokenness held back by the fact that there is no other INT full casterfor wizards to multiclass with that has spells the wizard doesn't already have.
5e multiclasses for spellcasters is powerful within the levels people actually play.
No. A completely new untested system has a high chance of being broken. Furthermore, your whole notion of the spell psion being broken rests on the idea of WotC would allow broken combos, not realising that they would be broken. And if they don't realise that, then they are not gonna realise the potential balance issues in a brand new system either.
No. My premise is that they
LIKE the broken combos and purposely design to give classes (especially the wizard) more spells.
With a new system, their favoritism would cloud their judgement and cause them to cancel broken ideas as seen in the Mystic. Because if they create a new system and it breaks, they will recieve the blame. But for many fans, you can't break the wizard and anything is allowed.
The cleric also has some favoritism and that how the OP Twilight and Peace clerics just through QA.
Raise awareness of the multiclassing and spell sharing being broken, as that's the root of your actual (or at least stated) issue.
It's
Spell sharing Feats
Multiclassing
And
5e Spell sharing and Slot stacking
I've complained about it in every book thread and every UA thread. But it's just little old me.
I have my big "I told you so" sign ready for when 2026 comes around and DMs are crying about uber-wizards again.