I'm not sure the top guys will do this. Is Deandre Ayton the 1st pick in the draft if he was in the GLeague instead of at Arizona last year? I'm not so sure. Playing against grown men is a lot different then playing against inferior players that are your physical equal. Even the GLeague is filled with vastly stronger and more physically developed players than a guy like Ayton. Sure, Ayton is more talented, but you can't just disregard the strength discrepancy either. Physical strength matters a lot and a skinny 18 year old kid just can't match up to a 25 to 30 year old professional. On top of that, the GLeague is a much longer season. That longer season will add a lot of physical strain on a younger still developing body greatly increasing the risk of injury. In addition, the GLeague has much crappier travel conditions. I mean a lot of the GLeague teams still ride the bus to games and stay in less then stellar hotels. A big university travels and stays in style.
That said, the money is nice. 125k from the GLeague is a good amount, but they can also sign endorsement deals, get paid by agents, etc. so they can make a lot of money in that year, which might outweigh some of the negatives and risk.