Wigginton and Horton-Tucker were able to leave early when they did because them going pro coincided with the NBA making the D-League into a serious AAA system. The NBA expanded the D-League to have one affiliate for each franchise (so NBA rosters went essentially from 15-man to 30-man constructions), improved the contracts offered to the players, moved the affiliates closer to their home base, and invested more money and seriousness into coaching and development at that level.
All these contributed to a windfall of more roster spots and good contracts being available for guys on the margin, like LW and THT, so away they went. But I imagine the system has already spent that windfall and established a new equilibrium by now. Foster still might be good enough to draw an NBA team's attention, but he is going to have to do it against a settled tide instead of a rising tide from a few years ago.
I would be perfectly happy for #10 to hold down the 5 position for us the next four years, be the perfect big to fit into the 4/1 we like to run, and go out on top with us discussing if we should be putting his number in the rafters someday. You know, if it happens, but keeping him around if he is really that good would be difficult.
Are G-League contracts "good"? If you're not a two-way player, I'm not sure those contracts have improved. I think THT is a two-way guy, so he's making a decent amount. But I doubt Wigginton was making any money, which is partly why he signed in Israel. In my opinion, he made a big mistake leaving ISU early.