I can see the logic to it, even if obviously the fanbase disagreedI might in the minority on this opinion, but I don’t expect to see any new Megas in any future games and a part of me doesn’t actually want to see any new Megas either. A lot of people seem to forget this, but base Sun & Moon didn’t have every Mega at launch, and Let’s Go obviously only had the Kanto Megas. Playing devil’s advocate I do think it’s incredibly stupid that only one actual Kalos Pokémon gets to use the generational gimmick of its own generation (and even then, said Pokémon is a Mythical) but, going back to your point, this artwork looks super good and I think a lot of these are actually better than their “old” official artwork from Gen 6.
…no, but like seriously, only one Kalos Pokémon ever got a Mega? Really? I hate Sword & Shield almost as much as I love the Ranger games and even I can admit Galar at least had the forethought to give multiple Galar Pokémon G-Max variants. How bad do you have to screw something up for me to say Sword & Shield did better than you…?
They clearly wanted to focus on new forms of existing Pokemon and specifically focus on Gen 1-4. Likely felt 5 was too "new" for such a super power up and Gen 6 would stand on its own. ORAS focused primarily on Gen 3 but threw a few bones to the other gens; Audino who knows why (if nothing else, it is a "weaker" gen 5 mon to grab?), Diancie since it's get the gem motif and movie to show off and being a mythic acts as extra incentive to trade it over/redeem it in ORAS
G-max (& to a lesser extent Z-Moves) likely benefited both from time to think about these things and also not being the same kind of super mode, so no issue giving about half the G-Max slots to Gen 8 (all other but 2 being gen 1 being its own issue, mind, but we digress). If the mega concept was introduced today it wouldn't surprise me too much if they still decided to put the bulk of the focus elsewhere, but maybe we'd get them for starters or legends (in the DLC, perhaps, if nowhere else).
anyway! I would definitely expect new megas with Z-A, they're reviving the mechanic and bringing it into the core series battling by way of Champions -which I would also point to meaning Gen 10 might even get it- so expanding the pool a bit would make a lot of sense even if future gens opt to not continually tie themselves to this mechanic and adding to it forever and ever.