I think it would make sense to make Sunken Temple a raid. If you think about it, BFD was pretty unpopular as a 5-man dungeon, so making it a raid means more people will end up seeing it. Kinda similar with ST, except until the class quests come out.
As for SM, I hope they only convert Cathedral and leave the other two as regular dungeons.
I would imagine with SM they will inter-connect them all to make the “raid”. So gy, lib, arms, cath all as one with the bosses of each being the raid bosses.