She can still get a monthly stipend from USA Swimming and can keep USOC and USA Swimming bonuses associated with Olympic medals (source: http://www.usatoday.com/story/sports/olympics/rio-2016/2016/08/02/paying-ncaa-college-athletes-at-rio-olympics-kyle-snyder-katie-ledecky/87709714/), even if she can’t sign endorsements. That won’t add up to a huge amount ($25K from the USOC for each gold medal, allegedly another $75K from USA Swimming although that’s not mentioned in that article) compared to what she could earn but it’s still a lot for a 19-year-old.
Another consideration is that a number of young American female swimmers who’ve chosen to go pro rather than go to college have subsequently had their career go in disappointing ways. I don’t know that there’s any clear, agreed reason for that but I would imagine if you were a swimmer it would be a consideration.
Also, the NCAA rules are stupid and should be changed.
Edit to add: I’d wait to go pro.