That's exactly what would happen.
As it is, on WHM, the longest queue wait is for Mainstream scenario (typically about 8 minutes), every other roulette is instant to about 3 minutes. It's always waiting for a tank, and part of that is because tanks don't like to tank some content, so it's easier to avoid that content by queuing only for content you want to play.
When I started leveling Paladin, despite having everything up to Haukke unlocked, every single time... Satasha or Tam Tara. Even on WHM... Satasha. Like, fine it's a faceroll. But there is no way for a "new tank" to not get a terrible zerg party who leveled off PotD, who rips hate off the tank with a single heal or DPS action. Like it's literately the worst thing ever to try and roll a tank now because people all learn these bad habits from PotD, or worse you get a raider healer who tries to DPS everything as fast as possible and thus rips hate off you.
It still would have happened. We don't need yet another sword-wielding tank.
What we need at some point is a tank that is easy to play regardless of the party configuration. The current tanks, for lack of a better explanation are "mitigation", "provocation", and "damage sponge", which suit different play styles, but yet DPS and Healers can rip hate off them easily until they have gear parity.
SE could also solve this by making ilevel sync to the lowest ilevel in the party. That way at least a tank with ilevel 55 equipment who is doing level 50 content is actually able to play that content without every single action by the other party members ripping hate off. The first time I tried to do any dungeon as a tank, the Healer wanted it to go faster so they just kept pulling more mobs. Stop doing that. Why would I ever want to do the 1-49 content as a new tank ever again when every experience is awful? I'll just go level it in PotD and not learn these dungeons.
It's incredibly easy to level a healer, because your gear as you level will not rip hate off a well geared tank. But a tank is forever chasing mobs around, and sometimes can't ever get that mob back since Provoke has a cooldown, and you can't just keep spamming flash after every cure.
Everyone, do yourself a favor, if you want more people to play tank, play at the pace of the tank, not the pace you want to play at. Especially healers, if you're ripping hate from the tank, stop doing what you're doing.