Well it's not difficult math. If you end up UDFA and sign to a practice squad for league minimum ($450K) when you could have posted a solid senior year, been a mid-rounder, and signed for a few multiples of that, the better monetary play is the additional year in college.
There's a big money difference between the rounds. 1st rounders get multi-million dollar signing bonuses. You could argue that if you got a $4M signing bonus and one season's worth of base pay once in your life, you're set forever as long as you're smart with that.
Say a guy makes $55K a year at his job (we're talking non-football player now). Nobody is gonna trash talk him for that. Solid wage. He'd have to work for almost 64 years to earn (before taxes) the 3.5 million the 31st pick got last year in signing bonus alone.
Risk of injury is there, but there's not a good way to quantify that. You can just as easily blow a knee on a practice squad as you could in college. If you're undrafted AND coming off a bad injury, now you're really climbing uphill.