If they don’t really have system mastery, then there is nothing to lose, but the new presentation might help them. Go for it, or at least let them use the new books and mix and match, it really isn't much of a big deal at all.
It is co-branded with a Forgotten Realms book already, and WotC has decades of proof at this point that making specifically Forgotten Realms products that can be repurposed easily will get them the most money.
At any rate, I expect there are elements they haven't hinted at yet, and that they...
Nope, I make no such assumption at all since the pr view images show us material that was not tested (4 Backgrounds, 3 Feats, and 3 Spells at least last confirmed).
However, for Subclasses and Classes they have not released anything that did not go through UA since March 2020 for Wildemount...
In what basis do you day that? They are specifically being sold together as a pair with tied titles, and the Adventure tied to Arcana Unleashed takes place in an area WotC specifically decided to call the "Arcane Empires" a few months ago into he new main FR book.
A subclass is like 2 pages, a background is half a pahe, that plus some Spells are what we have confirmed for this book will not take up the whole page count by a long shot.