That's probably a valid concern, but it's at best a fringe requirement, I would suspect, compared to the huge number of people who would want a snappier browser. Electrolysis provided some benefits here, because less parts of the browser would run in the same processes and threads.
Unlike the quip about working on Rust above, this is a matter of prioritization of effort. I would not qualify myself as experienced enough to make the decisions to run a project as large and with as many users as Firefox, but I don't think that Mozilla is wrong here.
I and about half of the people I know regularly test the limits of how many tabs they can keep open. The other half can't imagine doing so. I suspect this is a sharp bifurcation of use case that the project leads have found themselves on one side of, not a "fringe requirement."