I assume this voluntary retirement offer will be something like 2 weeks of pay per year of service, plus maybe 16 weeks. And you get to keep your unvested stock. If that's not too far off, you'd expect a principal engineer's gross expected value to be something like 800k from such a move. It seems like the net opportunity cost of accepting the VR would play out like:

1) you lose 1 or 2 million net, in the scenario where you would've otherwise stayed working 4 more years. 2) you lose ~200k net, in the scenario where you get another comparable job, but the job search takes 9 months. 3) you come out ahead if you were about to retire/get laid off anyway.

So, I don't think this plan is going to eliminate a random sampling of senior folks. The folks who accept this offer will tend to be ones that don't super enjoy the work itself, or ones that anticipate bad rewards or impending layoff.

In other words, I doubt it will be a sweet enough deal to entice the already-rich, high performing principal engineers, or the passionate nerds who are just there because they want to be.