Tenure was sometimes complained about, even back in the 1970s, as tenure could make it hard for new talent to get into the school's teaching positions. Not enough tenure track positions available and, at the same time, folks would say it's because they have to keep the dead wood; or at least what's perceived as the dead wood. When the institutions aren't growing and hiring, there isn't enough abundance to protect tenure AND provide a place for the new talent.
Tenure was good at protecting professors from the political whims of legislators who could turn off the spigot of salary if tenure didn't exist. At the same time, I think it was designed in a time when schools were growing and hiring. When growth slowed down, it became a choice between preserving tenured staff versus having room for new people in the system. Unfortunately turning into kind of a zero sum situation.
My thought about the term deadwood is related to the concept of ineffective rather than age, but I can see how it can be interpreted as age.
Effective and ineffective are somewhat in the eye of the beholder which is one of the reasons for tenure. Politics that's different than some legislator sitting on a funding committee could be defined as ineffective. Another term related to deadwood is boring. A professor who is not entertaining. To teach, I think it's important to hold the attention of students, but even that can be a matter of opinion. I've never been a teacher, but I have been a student; years ago.
As for the term "deadwood," I can't help but think of a joke that was popular during my college days. It may not be politically correct, but if someone thought a class was boring, they would often say, "the professor has already died, but they forgot to bury that professor."
It would be nice if there was enough jobs for both the aspiring new and already existing members of the faculty.