Are the incoming acquihired engineers that much more worth than the ones you already have? There are certainly exceptions but by and large I can't imagine this being the case.
There do need to be two conditions met to bring in the big bucks though: 1) you need to perform, such that your value to the employer is > $1M, and 2) they need to be afraid of losing you. Many employees fail on one or both conditions. They either slack off or don't work on the right things (so they're not actually worth that much), or they're "company men" who will unquestioningly stay with the company regardless of whether they're compensated fairly.
Definitely a lot of employees do sorta check out while their stock options vest, but not all. From what I've seen, maybe 50% of the employees in an acquisition aren't going to be worth it. But the 50% that do continue to work hard and assimilate well into the acquirer's culture tend to add a lot of value - many of the best coworkers I knew in my last job were there through an acquisition. I could easily believe that they're adding > $2M value amongst the 50% who continue working hard.
EDIT: Since I am being downvoted, I thought I'd provide some data to back up my claim. The average total compensation for a Senior Software Engineer in the US at Google according to Glassdoor is $267,413.