It typically makes content easier not harder to roll with one tank
There's so many fights in this game that have some degree of off tank swap mechanic but the damage of those mechanics is so trivially tuned a main tank can usually just eat it. and ignore the swap.
O4n was one example some groups would swap after x number of stacks. some would just plough through the whole fight even on week one. The swap mechanic was so loose that it could be ignored.
And even going as far back as coil, premade groups were running 1 tank for t5, t9 and a few others because taking a 5th dps made the run easier not harder.
The 2 2 4 party composition is the cause of many of the games problems though because it's built so heavily around balance. Thus doesn't allow jobs to stand out at all. Yet the devs also push jobs into pure roles which simply don't fit the dps meta.
Whm is a pure healer for example. But because of balance sch and ast can both heal well enough to clear content. it means the extra healing power a whm had was worthless to groups and thus whm was side lined in favour of sch / ast that bought more dps to the group
If they scrapped the fixed 2-2-4 comp though that pure whm healer might be powerful enough a party that they only needed one and thus that whm would offer something usefull to a party even as a pure healer.
Or a pld always concepted to be a defensive fortress but again balance says war and dark must also be able to tank so all that extra defence a pld has isn't necessary. however if you scrapped the 2-2-4 comp it could be that defensive fortress tank enough he could tank the whole raid Single handedly but a warrior might need another war or drk. To take some of the strain.
but the problem was the other tanks can tank well enough and they bring more dps. so A pld just isn't useful. But ya know balance has destroyed job identity and that defensive tank is now arguably the most kick ass offensive tank in the game. Because of a 2-2-4 comp and heavy obsession with balance. Which has destroyed job identity in many cases.
So if you really want to be radical scrap the whole 2-2-4 thing and let players build a comp they choose. Be it 1 tank 1 heal 6 dps or 2 tanks 1 heal 5 dps or 1 tank 2 heals 5 dps. or any combination under the sun.
That would be radical and allow for some solid job variation and identity. Then they really could have a viable pure healer class or viable pure defensive tank class. which would also gives players a lot more choice in playstyles those healers / tanks that don't want to dps then don't have to they have roles to play that suit pure heals or pure tanking sort of thing. would also open up the door to like pure support classes as well...