It's a bargain I expect most people would take, even with what I'm about to say next, but I also predict this is going to be a situation rife with abuse, with employees being pressured to "work extra hard" (eg, live in the office) during these two years, and plenty of people being fired for under-performing (eg, not living in the office) so they can avoid paying out the bonus to as many people as possible. Also, most new hires will have the impression that another 1.5M 2-year retention bonus is right around the corner, but won't actually get one.
If you do that (to that extreme), then you guarantee they will all leave after 2 years, and you have another retention problem. That’s just bone headed in my opinion.
Depends on the strategic goals. Plenty of companies have succeeded running high-churn operations getting a few good years from employees before they burn out and take a lower-stress job, maybe not even with a competitor (because they don't want to jump on the same treadmill at a similar company), or even just dropping out to raise goats.
