You talk about a kid who's in D3 commenting on why kids in college don't go pro...not a whole lot of correlation there but I will address both areas.
I would agree that the majority of good juniors who go D3 get weaker or stay the same. This is simply due to schoolwork. You can't play for 3 hours, condition for an hour and then study for 4 hours. Just not enough hours in the day. You need to be an exceptional individual to be outstanding in both school and tennis. I would bet the majority of your really top D3 players don't have great grades. At schools such as USC, Duke and Vanderbilt (academically strong D1 schools), there are watered down classes that the athletes can take. These don't exist at Chicago, Amherst, etc. Every class is challenging. You rarely see a kid who truly blossoms playing D3 tennis and it's because tennis is only their mind when they are on the courts. I don't think it has anything to do with weather, as your friend mentioned. Middlebury played Amherst in the final this year. Both in New England. All the top programs in the country have readily available indoor courts so I don't see this as an issue. Most of your good students, including D3 players, have internships during the summer, so I would say they actually become out of practice when they aren't in school.
As far as kids not going pro out of college, I think it's just pure numbers. There are 6 (not sure?) Americans in the top 100. 2 of them played college (Isner and Ram). Maybe 1 American who finishes school every 4 years will break the top 100. Your chances just aren't very good. If you want to talk about the minor league pro circuit, the summer US futures are flooded with college players during the summer and former college players all year round. You all see a decent amount of former college players in Challengers (Michael Yani and Robert Kendrick). Addressing whether D1 players get better, I think it just has to do with work ethic and level of caring about tennis. Guys like Alex Clayton (Stanford), Jared Pinsky (Duke) and Kellen Damico (Texas) were better when they entered college than they are now for whatever reason. Then you have Josh Graves (Northwestern), Abe Souza (Illinois) and Drake Bernstein (UGA) who were 2nd or 3rd level juniors that absolutely blossomed during college while playing indoors. Granted they aren't good enough to be top 100 pros, but they definitely got a whole lot better during college tennis. You get out of it what you put in, so I would argue that whoever gave you that advice is incorrect. It really has nothing to do with cold versus warm weather, it just has to do with how much time you devote to tennis and whether you're indoors or outdoors I don't think matters.