There should be no life time tenure for any of them.
Maybe give them 5 or 10 year contracts with escape clauses.
The problem is that this leads to "trendy" research. You see that in the sciences today. Every math department wants somebody in biomathematics or some such thing. It is, at best, mathematics with biological applications. At best. At worst, it is a polite fraud.
With tenure, there's enough problem with trendiness. Of course, a lack of tenure would benefit us in the humanties now because it would break the back of the 60s crowd and the younger faculty that they hire under rigid standards of political correctness.
And we also need to kill PC in the accrediting agencies.