Well first off, Rondo should not be playing 38 minutes per night. He needs a backup and if he had one I'm sure he could reduce minutes.
He can also shoot some, so he could play alongside Rondo. Not that it's ideal, but if he can defend, and he can shoot, they can play together.
And to your last point, everyone would have said the same thing about Rondo when he was drafted. He is coming out of Euro basketball so there's a fair chance he will be underrated. No guarantees of course but from what I've seen he has upside certainly.
And finally, if we are going into a rebuild at some point, taking who you see as the most talented player is the smart move. It's all about building assets.
For the most part, I don't disagree with you.
However, I think Rondo is always going to be playing 36+ minutes a night until he breaks down. That's just the player he is, especially in the playoffs.
Second, even though taking the BPA is almost always the way to go, especially if you are rebuilding, you have to consider the opportunity that the player will get to develop. For the above reason, Schroeder isn't going to get the same chance to develop as a big man with real talent and physical gifts. Minutes are truly the "royal jelly" for prospects in this league and a PF/C would be more likely to get them.
I also disagree with the idea that because we have Fab Melo already we shouldn't pick up any more big man projects. There's no guarantee Melo is going to pan out. That, of course, is the nature of projects. Also, even if Melo does pan out, is he going to play 48 minutes a night? Chances are, the best we can hope for from Melo is that he is eventually capable of giving us 15-20 solid minutes a night at center. There's still plenty of opportunity there for somebody who can also play at center and complement Sullinger in the post.
I definitely view trading Rondo in the near future as a high likelihood, so securing an heir is not a bad idea. I just don't have much confidence that the 3rd or 4th rated point guard in this draft is likely to be that guy.