The idea sounds like a totally different game.
As far as your gripe goes, if it’s a Mythic raiding guild I’ve experienced that and started recruiting for people who could dps raid and preferred tanking keys. You want backup tanks for call outs and dps that can also flex heals anyway. If they’re good enough for mythic raid it shouldn’t be a problem. That or they could design raid for 4 tanks, that’s a much simpler solution to the stated problem. It’s a leadership/recruiter problem for not planning ahead, imo, not a major design flaw despite being inconvenient.