Then you'd need to create other design elements to make them more distinct defensively, as they would just be near carbon copies without the variation of sustains used to define them more.
Sustain is fine for tanks to exist, it's only a problem when it starts invalidating the existence of another role. Stormblood Warrior for example had strong access to healing but at cost of damage, and even then you usually still wanted a healer to help you out.
