It is not possible for all healers to heal 100% of their possible globals. That isn’t a design decision, it’s simply an objective fact. At some point a healer will reach the threshold that they can completely negate all incoming damage on their party, and a single point of healing more does literally nothing. For the healers who have 100% GCD saturation with the design, all healers who are better will have free globals and all the healers who are worse will be incapable of keeping their group alive.
It would be possible for there to exist some activity other than dealing damage for a healer to fill that time. But such mechanics don’t currently exist, and the options for healers are to contribute damage, knowingly commit massive amounts of overhealing, or do nothing until the next damage event.