Speaking as someone who knows the deals of a few NCAA stars who have gone pro in the last several years, some of the numbers thrown out here are incredibly low. I will not try to guess an exact yearly salary or contract length, as there are too many factors to consider, but, in my opinion, the idea that she signs for any less than $250k annual base salary seems far-fetched. Although my knowledge is more focused on what several male athletes have received, I have a few female references.
Additionally, one name that comes to mind for comparison is Candace Hill. Similar to Sydney, she was a star very early (what most would consider 'massive success' at the age of 16). However, Candace decided to capitalize on that and sign a 10 year, multi-mllion dollar deal with ASICS while the hype was high.
While Candace did have the advantage that came with signing with a 'lower-profile' company like ASICS (signing the largest contract ever for a female track athlete with the company), the broad assumption is that Sydney will probably end up with Nike or Adidas. This, plus being slightly older, will mean she probably doesn't sign a deal of equal length, but her proven success at the collegiate level and posting of world-leading times will give her a boost. Add that to the things people have already pointed out in this thread (looks, social media following, potential longevity, etc.) and the fact that she is quite well-spoken and handles herself well on camera, and McLaughlin will signing a very lucrative deal some time very soon.