The big disadvantage is that you now have another status level above you that did not previously exist, which should you push down the upgrade list below them. That is what will happen.

That will include the Marriott Platinum members grandfathered in, with no means for an SPG member with an equal number of nights and spend to do so.

This is the same thing United did to its Million Milers while they gave Continental's similarly situated lifetime members a bump up. It unnecessarily creates some very, very vocal and unhappy people. Take a look in the history of the United board for the numerous threads. It wasn't pretty.