This has been peddled by the community for a decade now (or six years for M+), but the simple fact is that the community isn’t able to handle this distinction because of the constant need to BiS.
Even if you introduced separate sets for M+ and raiding right this moment, you’d be inevitably faced with the BiS issue. People will still require the 2-set/4-set tier bonus from M+ in the raid and vice versa if it is a substantial DPS increase.
If you make the tier set effects work across different content (e.g. an unlocked system) this will 100% be happening and we’re back to the old problem of having to run M+ every week multiple times even as a raider.
If you make the tier set effects locked and only work in their respective content, we’re back to many people having to work on two different sets to be effective across two kinds of PvE content that (due to the vault) work together. So you get double the RNG for the same amount of fun.