>I have come to believe that the main purpose of hiring scientists in academia is to keep knowledge alive and have it passed on to future generations.

Then these scientists should be devoted to producing textbooks and courses which can then be taught to non-research students. Yes, all knowledge about the scale of what a single individual knows (and keeps on their shelves, hard drives, etc) is embodied as communities and traditions, but we still get far greater redundancy of that knowledge from teaching it as undergraduate or master's-level coursework than from passing it down only via research mentoring.

If 25% of the population gets an undergraduate degree, 11% or so gets a postgraduate degree, and only about 1.7% get a PhD, then we need to be embodying society's knowledge among the larger cohorts for that knowledge to survive. We can't afford to live in a world where only 1.7% know how things work.