And there would be no chance of transferring money between teams. I'd say that would have been the biggest factor in banning sales between 1st and 2nd teams, not the players themselves
It's good that you actually brought up money transferring. Because what's wrong with transferring money between two teams you own? It gives people with Utopia teams an advantage in finances over those without, right? It's no different from your suggestion really. It allows players to use real cash for a feeder team to boost their main team above those who don't spend money. That essentially makes it P2W.
But honestly speaking, I love the idea of a G-League or feeder team. Just that it shouldn't require Utopia teams or any paid features.
One way to achieve that is to actually have something similar to Hattrick's "Youth Academy" (since a lot of us "migrated" from Hattrick back in season 3 I'm assuming a lot of users should know about it). It's something like the gym and training court - a feature which requires weekly upkeep (that comes from your team's account, not your real world bank account). You can also join private leagues consisting of feeder teams (where winning just gives you bragging rights) with these feeder teams, and training will come from those minutes. You still need to pay the salary of all the players in there (and maybe even recruit the team yourself, unless having a default 5 man roster when you set up the feeder team is a good idea), but you're free to transfer them in and out just like between the NBA and the G-League.
This not only allows you to keep your pirzed trainees (and train them), you can also ensure your 60-70 year old players get game time, and those who don't have small rosters can keep the rest of their players' game shapes up during the allstar week (and people might no longer keep suggesting about paying 50% salary that week).