My only problem with this is that during new expansions a legacy raid would feel like more of a chore to go to than dungeons for your average M+.
The traveling is more forgivable when they're really rare Timewalking Raids you don't do as often and the rewards are greater, but if the rewards are going to go down to just dungeon mythic level (especially during a time of less dungeon loot in general) and come up as frequently via Keystones during the M+ season it feels like the Dungeon Finder teleportation to dungeon would be necessary to make doing them feel like less of a chore. (Like how they did the Molten Core anniversary, which I liked.)
Some legacy raids like Ulduar where there's just so much running around may also be kind of annoying in that regard. If you teleported to say just the first boss instantly in those kinds of cases that would be nice.
I feel like some raids like Onyxia and Malygos and Ruby Sanctum and Sartharion are really short comparatively and I would fear longer raid wings would be seen as chores or not worth running or dead keys by the community.
It would be cool to see the legacy raid hard modes turned into extra difficult switches when on Mythic - that might be wild.