That seems like the opposite. Why would someone with high market value stay in one place? 2 years is basically optimal - you vest 50%, maybe collect a promotion, do some good work and learn a lot, and then get to move on for another solid bump/ promotion and a new set of stocks.

I expect the people with low market value to be the ones sticking around labs for long periods of time, they don't have the option to move and they aren't getting poached.

It's incredibly hard to do good, novel work in 2 years for engineering. You'll likely not learn much either.