There is both supply and demand. We know the supply of modern postally-used covers is way down from years ago, and declining. The question is future demand. Even if the supply is low, future demand may be even lower, which hurts value. Despite low value, during the next century I suspect there will still be people happy to have a cover bearing a forever stamp from 2020 if it's one of only, say, 10 known to survive. I would, and suspect most here would.
This argues for saving whole covers, something I have not done due to the unwieldiness. I suspect a forever stamp with a contemporary spray-on cancel will be as interesting as the whole cover because the cancel will identify the period it was used. Next-gen spray-on cancels are already being tested, and likely will supplant the current version this decade.