An argument against a union at MIT came from un-tenured graduate student Kevin Wang : https://thetech.com/2021/10/21/why-we-do-not-need-a-gsu
TLDR: he didn't think paying the 1.44% annual union dues (~$620/year) is going to be worth it. He based this on comparing MIT salary increase history to other universities with and without unions.
Also, KW was previously in Germany & UK so he's presumably familiar with the stronger worker unions in Europe.
The MIT union vote was 66%-for and 34%-against. The 34% have various reasons to not join a union but I think we can guess that many of them think the $620/year salary deductions will not be worth whatever benefits the union can negotiate.
I'm not saying KW is right but I think it's more constructive to consider the opinions of MIT grad students who actually vote (e.g. Kevin Wang) rather than your synthesized quote from tenured professors. The collective bargaining unit will negotiate with university administrators (not the tenured professors) who set salaries etc.