Learning is a process. Marvin and RJ are helping the young guys learn. They still have a lot to learn. Do you really think experience/mentoring are of no value?
I think it's reasonable to think that succeeding in an appropriate and well-defined role and winning games helps players learn better and faster. Players absolutely can regress when placed in poor situations where they're bound to fail. I think it's ridiculous to put losing games ahead of development at this point, especially since this team won't be as bad as Milwaukee, Philly or Orlando unless they unload their young talent. For the most part, the young guys have been given ample opportunity and responsibility to learn and grow. I wouldn't mind seeing Alec take a few more of RJ's minutes, but that's a minor gripe (and Alec is overrated on this site). It's unfortunate that you've already written off the young players on this team.
With that said, if the Jazz can acquire something for Jefferson, they should definitely consider it. Rush has been playing better of late and Alec has shown improvements playing off-ball this season. Like everyone else on this site, I don't know what goes on behind the scenes, or how much of an impact RJ is having on the young guys. He and Trey seem to have a good relationship on and off the court, but again, I don't know the relative value of that and whatever assets you can get for RJ. Like jeditiger, I think the draft will sort itself out, and the focus should be on how to best use and develop the talent currently on the team.