I think the concern comes from how they might end up changing and balancing the tanks though.
Without more information which we'll get from the next Fanfest, we only have a couple of quotes from Yoshida about the direction in which they're taking tanks.
One aspect, is they're wanting to remove the current "Triangle" of tanks, where you have your MT (WAR), OT (PLD) and the class no-one really wants, the Switch (DRK).
How they go about removing this "Triangle" can be done in a number of ways, not all of them good.
For example, it was mentioned that they might balance 2 tanks to be MT's and 2 to be OT's - The fact that this was specified, rather than saying they'll balance them to be able to perform both roles but have some strengths in one role, can mean they might make it so that the MT's don't really perform well in the OT role and vice versa.
Like, ideally all 4 tanks would be like DRK, being able to comfortably play either role and still bring enough to justify their use. With maybe a few extra unique aspects that make them a bit better in one role over another. An example would be WAR being slightly better at being MT because Holmgang is a low CD skill that negates a Tankbuster, something that is unique to WAR but that wouldn't mean they're so far above a more OT focused tank like say, PLD, that PLD wouldn't be seen as useful in the MT role.
But according to Yoshida's quote, they're currently looking at either making 2 MT's and 2 OT's OR looking to make all 4 tanks able to perform both duties.
This leaves open the concern where they might make it so that each tank only can really fit into 1 position come end-game raiding. Which would suck if someone really liked a certain Tank due to aesthetics, lore or general gameplay, but preferred the role that it wasn't balanced for.
Which isn't a particularly out-there thought, given the state that WHM and MCH have been in for a while in regards to end-game raiding (I.e. Where both of them are just plain inferior to other options in their role and thus not wanted)