Probably, yeah, they would, and rightly so - why buy expensive nee books if already own what’s in them? In theory the sweet spot is creating enough nee stuff that old fans buy it and still doing a mechanical update for brand new people. 
Any complaints over “psionics is magic” seem overblown to me - we doing know yet how careful or stupid they’re going to be with that idea (though, obviously “pretty dumb” might be a safe bet). Divine magic has always been a kind of magic, but its obvious and demonstratively different from arcane magic. And, psionics has always been called “mind magic” in the Forgotten Realms with absolutely no mechanical effects on its workings. Psionics was already a kind of “magic” in 5/5.5e, this could just ine of WotC’s normal (if pointless) conceptual clean-ups that changes nothing about the setting itself (and, don’t lie to yourself, 3.5e psionics was functionally about as close to being magic as anything could be but was still defined as being “different then magic”).
I have all the faith in the world that WotC will screw this up, but psionics being a kind of magic isn’t inherently a change that really HAS to mean anything mechanically.
IDK about changes to the bard, but it probably won’t be any more or less disruptive than 4e DS using warlock as the replacement for the templar class (which i had a lot of issues with, but - in all fairness - didn’t BREAK the setting).
I will say “Why don’t people distrust psionicists, if psionics uses many of the rules for magic” might simply be solved by saying “because there’s no possibility of defiling.”
If you make defiling a mechanically beneficial thing (free upcasting is a common suggestion), or even a mechanical trade-off (eg upcasting but penalties on Charisma checks) and psionics simply cannot do that, then that’s plenty of reason to distrust wizards.
A preserver, in this case, might be very similar to a psionicist. But if they choose to defile (which you might make a background feat, or simply a class option for wizards), then they destroy the world a little bit more. And while we see the “back end” and know, Doylistically, that psionics and magic use the same rules, the Watsonian people of the Tyr Region will know how psionics and magic work, and they work differently, and psionicists don’t destroy the land.
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Sure, but this still assumes that psionics and magic are somehow conflated in-universe, and we haven’t really seen any proof of that yet.
I have nearly no faith in WotC to get things right, but there’s nothing to say that the ideas of magic and psionics are going to be changed AT ALL even if the mechanics are harmonized slightly more than they had been in previous editions. Psionics was just another power source in 4e, and it was “works just like magic except it doesn’t” in 3.5e.
The counterargument for “Boy, i wish psionics worked COMPLETELY differently than magic” has always been “Ugh, why make it so different.” We did 5.5e psionic “mages” in several adventures so far and the sky hasn’t fallen yet.
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Not my intent; while the view of the game mechanics might make it clear they’re essentially the same, Agis and Sadira don’t know the game mechanics, just how it works in the world. What they do know is that Agis can be a psionic and that’s fine, but Sadira being a wizard is a problem.
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I 100% agree. Mechanics can, but don’t have to, inform the setting.
Hmmm. I was actually more thinking in terms of “how random people observing somebody create an effect recognize if they’re seeing psionics or magic”.
If it rips at their soul like a thousand little needles, while ropes of green energy flow towards the caster, it’s magic.
If it doesn’t, it’s psionics. Bonus points if it looks like they’re trying to pass a big poop.
They also have to put a couple of fingers against the side of their brow.