Game Freak etc. have contracted all the other side Pokemon games off to other companies many times.
That was The Pokemon Company that did it, not GameFREAK itself. GameFREAK doesn't self-publish except in rare circumstances (otherwise SEGA wouldn't be the publisher of Tempo the Badass Elephant ;) ).
Officially The Pokemon Company serves as publisher for all Pokemon games even the mainline ones. GameFREAK can only publish their original IPs or get another publisher to do it (like SEGA with Tempo).
Yes, GameFREAK recycles all the time, but resource management is still a thing even on more capable hardware on Switch. Something had to give eventually since GameFREAK stopped making third versions and gone with remakes or sequels in the third version's place. If Diamond/Pearl/Emerald get remade for Switch next, they'll be the versions with the missing Pokemon from Sword and Shield.
Or hell if Sword & Shield does well enough, there will be a Sword 2 and Shield 2 with the missing Pokemon. ;)
EDIT - And back to Mega Evolution, not every Mega was patched back into X&Y following Omega Ruby and Alpha Sapphire. That's probably going to hold true for Sword and Shield that any Pokemon that is in Sword & Shield won't make Sword 2 and Shield 2 or any possible Sinnoh remakes.