The problem with getting rid of tenure is that we have an up-or-out system in academia: you either rise to tenure, or you're fired. Permadoc positions would be nice, but if you eliminated tenure then academics would just become more abused, overworked corporate employees. Not because academic freedom would die, but because "if you don't publish X papers and bring in $Nx100 thousand research dollars this year, we'll get someone who will."