You can't hand-wave the uphill climb buttershot players have. It's extremely hard to shut players down in the modern NBA, and if you do, it's a much of a team process than that of an individual player's effort. I don't think there's a justification for Castle at 4 if you don't believe he can be a decent shooter in the NBA. It would be one thing he had some potential to be a real on-ball dynamo ala Collier where you can say, "Well he might have to have the ball a lot, but he's going to be a huge wrecking ball against opposing defenses which can get the others good looks". Even if you added Collier's driving threat to Castle, the shooting would still be a problem. But without it, you run into what's actually a pretty common archetype of passing defenders with size and handles but who can't space the floor. There aren't a ton of those guys who are very successful in the modern game. Bod provided a list of PFs with some wings who haven't really had careers you'd want out of a top-five pick -- even in this draft. You have to think Castle is slated to be better than them, because most of those guys are available this summer for cheap.
All that said, I'm not giving an opinion on Castle's shot. I don't know how bad it is or what the Spurs think about improving it. I do worry because the Spurs have had a horrible track record of improving shots since Kawhi and Green a dozen years ago. Bad shooters have stayed back and guys who came to the team as good shooters have regressed. I don't blame them for letting Chip walk, as the results weren't there. But moving on from him didn't seem to improve anything either. I'm a believer in the concept of "trust your staff or replace them", so if the staff says they can fix the shot, the Spurs should go with that. But it's a huge risk, for Sarr, Topic Castle, Buzelis, Holland, Collier, Edey, Dunn or whomever else. The Spurs can afford to let the shooting come along as their guys improve. They can't afford to draft guys who can't shoot and try to outsource that task to free agents. That's how you get guys like Forbes and McDermott getting big minutes.