The greatest gift Palm's WebOS gave us was the unified calendar (and the greatest curse was those ads with the lady on the rock). I agree that having too many calendars is problematic, but I think a handful of calendars, shown together, and shared as necessary, is the answer. I also am troubled by putting personal stuff on a work calendar, because I think that's a line that should be rarely, if ever, crossed. If you were unsatisfied with your employer and interviewing, would you put interviews on your work calendar?

Another commenter mentioned it, and I agree, there is something to running a calendar server of one's own, if one is capable of doing it. I've been trying to scratch this itch for years to little success, but feel like it gets closer all the time. Things like NextCloud make it even easier still, but lack the way I want to deal with groupware (service accounts). I'm hopeful that Apple Family Sharing can solve some of this for me and my partner as we both use iCloud calendars today, but haven't set it up yet.

Over all, I agree, and wish I knew the answer, but I don't.