Of course most college players won't play pro ball after college. There are thousands of college players and only about 400 or so roster spots available in the NBA and its developmental league. And as it so happens, many of those players that don't make it to the NBA end up playing (and starring) in the Euro and other top overseas leagues.
The NBA has been pipelining the top players from Europe and other foreign leagues for about 20 to 25 years now. In all that time and of all those players, only a very small number have had NBA careers of special note. And only one (Dirk) could ever really he considered a top ten player during the time he played. The vast majority have been washouts or fringe players, and only a small percentage have had solid NBA careers.
The fact is the vast majority of top level basketball talent comes and will continue to come from the American college ranks... and that's not even close to something that can be legitimately disputed. Heck, the Suns even signed one of the best vet players from the European league a few years ago, and in his one year here, barely looked he even belonged on the court with one of the worst teams in the NBA.
As for Doncic, I don't see how anyone can evaluate him as being so spectacular that he is the indisputable best prospect in this draft, simply because he has looked good playing in the Euro league. I am not saying he absolutely develop into a solid or very good NBA player, only that it's far from a sure thing. As has been pointed out, he is not an elite athlete, and at 18 years old, he is also sure to have a big learning curve playing in a league full of far more elite athletes than the one he has been succeeding in. I certainly don't see him as being more NBA ready than any of the better college prospects in this draft, and due to his athletic shortcomings, his ceiling is probably lower than many of them.
As the Suns rarely get a good pick in a draft that's as stacked at the top as this one, I am not sure at all that Doncic is a good risk at say the first or second pick, if we are lucky enough to get one of those. As I said, maybe he is not so much of a risk if we end up picking lower than that where other more high ceiling players are off the board. But the insistence that he is somehow a once in a lifetime prospect we can't afford to pass up is plain silly to me.