Is Dean-hood largely irreversible, and if so why?
To become a dean at a research university, you have to have some prior administrative experience. So, chances are that you were department head for 4 or 8 years before you become dean. By the time you step down as dean, you've been an administrator for more than 10 years. You may have been able to keep up some kind of research program as a department head, but that's almost unheard of for deans -- certainly not at the level where you would be able to keep a lab running, write grants to support your lab staff, etc.

In other words, when you step down as dean, you're in a situation where you likely haven't kept up with the literature in the field for a good long time, where you don't have any current grants, where you no longer have grad students and postdocs, and where you haven't been to a lot of conferences to interact with the other people in your field. If your plan is to go back into research, it's going to take a while to make that happen -- and it does, and universities usually also give deans a year of no teaching after they step down to get back into their field. Or, you can decide that your many years of administrative experience are useful skills and continue to use them in other roles.