Tenure is the one big incentive academia has going for it. Un-tenured academics work like dogs for peanuts (given their skill sets) to get tenure. You take that away, and it becomes "work like a dog for peanuts your entire career". Who's going to take that deal without the carrot of "do what interests you without fearing for your job security" at the end of the tunnel?

Streams of irrelevant publications are only going to get worse when "publish or perish" extends to your entire career.

(Not to mention that tenure is a good idea for, say, climate change researchers, or people doing potentially provocative demographic research, or stem-cell researchers, or anyone else someone influential somehwere might want to see silenced)