Much of education is based on manufactured scarcity - so for example, if a person is an expert on a particular discipline and is able to provide support to those who wish to learn about that discipline, access to that expertise is artificially restricted.
If they are employed in an educational institution (school or university), then access is restricted to those who are enrolled there - because that allows them to be more attractive to potential students (and then be selective in terms of either the "quality" of the students and/or the fees they pay). If they are operating independently, then access is restricted to those who can pay for access.
Now some would argue that these experts need to be paid for their work. OK, let's accept that they need to be paid - why can't they operate like a doctor in a public hospital, who is available to everyone who has a need? Especially in the case of public funded education?
Of course, this would lead to more demand for the person's expertise than they can handle. So we need a system of triage, like in healthcare, that determines priority for access. In healthcare, this is based on medical need, which is assessable. Educational need is much harder to evaluate, but what if the triage is based on who can best represent the interests of the rest of the community - so it's not just about using the access to benefit oneself, but sharing that benefit with the community (e.g. themselves being an "expert" who supports more people)
Would experts want to be part of such a system, rather than the scarcity model where they can potentially earn a lot more money? Only time will tell.
But even if they could spend 10% time on open access as a pro bono service, it would still allow for their expertise to reach a wider audience than it does now, and can especially reach marginalized audiences who presently do not have access.