i.e. If Google wants another engineer it can hire from outside or it poaches Facebook's(or another's in that club). In the second case Facebook (now needing an engineer) will then either poach from Google(or another in that club) or hire an outside engineer. The only way an outside engineer isn't hired is if Facebook/Google decide that an extra engineer is not worth the new higher salary (or the expected lower worth of them since they constantly jump ship).
Even if the pool is smaller than the current pool of engineers at the big companies it would still arguably be a "fairer" distribution of wealth because the profits are more evenly spread through the successful companies. It would also mean that as the larger companies produce less (fewer engineers to make things) there are more engineers that could be working on competing/niche products that are currently worked on by the larger companies.