This is what I found odd about Blizz wanting to turn WoD dungeons into M+ dungeons: it'd be way easier to convert almost any dungeons into a CM dungeon, but converting them into a M+ dungeon is way more work. I'd rather Blizz just bring back CM's as end-game content again. You could easily convert every M+ era dungeon into a CM dungeon, and you could appease all those people that just don't like the hassle of M+ affixes. Heck, you could go back before MoP and convert dungeons to CM's relatively easily.
Personally, I don't mind limited time stuff, however I was never one to get caught into the FOMO hype... so if I missed something, I didn't feel bad about it. I'd rather new rewards come out that are on par with the old stuff every so often to give people new incentives instead of clinging to old things. Nothing wrong with rewarding your loyal players with unique stuff, as long as you aren't exploiting it for player retention reasons. May be a fine line, but as an example: having the CM rewards for an expansion being available for two years doesn't qualify as FOMO in my mind at all, making it perfectly reasonable to keep them limited to the expansion. Having CM rewards only be available for really short periods of time (like how the Mage Tower was basically only going to be available 2-3 times a year for time walking until the recent change) is definitely in the FOMO territory.
All in all, it's a balance. If you make certain rewards limited by means that employ obvious FOMO tactics, that's bad. However, if you make no reward feel special and unique, that's also bad. You have to find a balance to avoid FOMO and apathy when it comes to your rewards.
If you want my opinion about what the real problem is... it's that Blizz hasn't been releasing interesting cosmetics outside of the in-game shop, as well as flooding the game with mounts/pets to where everything just becomes another number to your total. We've gone from "You have an epic mount? that's so cool!" to "...and that's mount 534, just need 66 more for 600!" I think there'd be less conversation about the FOMO aspects of rewards if there were actually more unique rewards for doing interesting things versus the current system of flooding the game non-special stuff.
One of the major reasons why people seem to crave stuff they missed in expansions past is because Blizz hasn't been putting out anything of that caliber into the game since. Heck, I think the WoD xmog for CMs was generally terrible for many people's taste, to the point where I couldn't get my guildies to even finish the gold CMs in WoD because everyone thought the xmog was ugly... guess Blizz should've put the mount at the Gold instead of Silver. Regardless, you cannot placate everyone, but I feel the best solution is to bring back the content with new rewards at least on par with the old rewards.