They may or may not show up again in future if they feel like it (I doubt), but I don't think we'll ever get more of them nor any comparable "pokemon specific" mechanic. At least not in the mainline games.
We probably will see Megas again someday, but mainly if they do show up they'll be in a future game centered around Hoenn or Kalos when we revisit those regions again in the future. Why those two regions specifically? Because Mega Evolution from a lore point of view is pretty strongly associated with Kalos and Hoenn and the phenomenon behind it is mainly taking place in those two regions where Mega Evolution energy is prominent and the Mega Stones were all discovered there.
Their philosophy with super mechanics these days is to rotate old ones out and create a new one introduced in every generation, and each generation's "super mechanic" from a lore standpoint is entrenched in that generation's flagship region's lore. Z-Moves were the super mechanic of Gen 7 and lore-wise it's strongly attached to Alola, Megas and Z-Moves were both removed from Gen 8 and Dynamax was the super mechanic of Galar, and we all know that's associated with Galar. Meanwhile, Dynamax itself was removed in Gen 9 and now we have Terastal, which is strongly associated with Paldea.
And we all know full well at this point that once we get into Gen 10 that Terastal will be out the door and a new super mechanic will take its place. And that super mechanic will be associated with Gen 10's flagship region.
With that said, when we revisit these regions again the corresponding super mechanics will return with them for sure, because from a lore standpoint each super mechanic's in-universe phenomenon is strongly associated with the region it debuted in (Megas are also associated with Hoenn due to ORAS being Gen 6).
They just had a shift in philosophy and decided that the supermechanic must be generic
As I said, it's also a philosophy of rotating new gimmicks in and old ones out every time. Z-Moves and Dynamax are both gone, the former was also dropped after Alola, and Dynamax was now dropped after Galar.
I think Terastal handles being "generic" the best, however, because it's so generic that no one gets special treatment ie a special version of Terastal, Z-Moves and Dynamax tried to be generic but also specific at the same time: you had generic versions of Dynamax and Z-Moves but then certain mons also had signature Z-Moves or special versions of Dynamax called Gigantamax.