How many seniors would realistically take another full year in college, especially those who weren't already planning a grad school year? Starters at top 10 programs? Especially those at non-public schools who might be paying a good chunk of tuition each year.

Do coaches want to keep their all of their seniors around when they have fresh kids coming in? Probably a select few from each senior class.

I think after the rush of emotions wear off, most seniors will want to graduate and move on. And I would expect most coaches would encourage them to do so.