Tenure cannot be taken away from those who already have it (barring malfeasance on the part of individual faculty members)...it could only be denied to future hires. That would make an academic career much less attractive to the best and brightest, and would do nothing about the proportion of leftists coming out of graduate schools.
Right now one problem with universities is the high proportion of courses being taught by part-time adjuncts and temporary hires...eliminating tenure would make faculty turnover much more common, without any increase in intellectual diversity, and probably make for an academically poorer experience for the serious students at a given institution.
For ex., at my school our department had 20 full-time tenured faculty about 20 years ago. Now we have 16, but two or three of them are really functioning part-time because they serve other "programs" and only teach one course in history and often don't serve on our committees, etc.
A "union cartel" argument would make sense IF---but only if---faculty salaries were increasing faster than a) inflation and b) other outlays in the administration. But it isn't close. Most big state schools have 2% salary increases. We have been lucky---we are at about 3%. Admittedly, our benefits costs have skyrocketed (as have all industries and companies). But Vedder shows a consistent DECLINE in the $$ going to faculty, even accounting for bennies. Instead, administrators, and especially "programs" such as psychological help, "special needs," English as a Second Language, and so on have driven costs through the roof. So viewing tenure as a "union card" is marginally correct, at best.