re: 20 years from now, will half of the NFL be SEC players?

I don't know about half but it will be represented well.

I'm looking at today's paper with the Rivals Top 100 (not saying it's the best, but it's what I have) and 30 of the top 100 have verbal commitments to SEC schools. This doesn't include those who have listed multiple SEC schools among their top picks. It may not be a great judge of the future (as few college players actually make it to the pro ranks) but it's probably a good estimate.