I think it's more along the lines of each prime plane/world has it's own magic source. Mystra is the one for Faerun, and not even the only one since Shar has the Dark Weave. The outer planes have their own sources and I doubt that source is a god.
And if this were true, one of two things would be true.
1) All planes of existence would have magic working differently. They do not, so this is not true.
2) All of the "sources" of magic would have to get together and agree to present magic in a unified manner, that works consistently between the planes. Meaning that they would have had to change to match the "new normal" of the Spell Plague after Mystra's death, because that altered magic across all planes of reality (ie, the game system changed) and therefore they must have had to meet and agree,
Also, considering how important Mystra is, I'm sure the "source of all magic for the Abyss" or 'the source of all magic for the Hells" would be mentioned... somewhere.
So, is there a third-party alternative psionics system available yet? This looks like an opportunity for someone to do what Dreamscarred Press did for Pathfinder 1e...
There are a few. I'm a fan of Kibbletasty's psion. Actually playing one right now, built to deal te maximum amount of damage, and it feels pretty close to the bar, I'm not OP by any stretch.
Read that section. The rule is, no "weave" = no magic. That's crunch and is spelled out in that box.
And you have to put that in quotes. Why? Because The Weave was created by Mystra in the Forgotten Realms. No Mystra, no Weave. No magic in any setting that does not recognize Mystra as existing.
So, you have to say "weave" to represent Magic. And that makes your statement "No Magic = No Magic" a tautology. You are just saying the same thing twice.
The Weave is a setting detail.
"the weave" is just a convenient model to talk about magic.
Then whether it was a conscious decision or not, you replaced the weave with something else. That's the only way per RAW that magic can continue to work.
Dude. No.
Guess what, I can make a world without any weave, and still say magic works. Unless you mean "weave" as in "source of magic" or "magical energy" in which case, yes, You need magic to have magic.
But claiming people who don't use the weave are subconsciously replacing it because the rules require the weave is just not how things work. Ever.