You speak as if teaching in-person is an apples to apples comparison to teaching online where online is strictly easier and of less value. Having done both, this is far from an equal comparison. Putting student value aside, the skill sets needed are different. A professor that can teach in-person AND online equally well is not easy to find. So unless the university is willing to fire their professors and hire ones that are better trained for online teaching, I see no reason for the professors to take some sort of pay cut.