There aren't enough good QBs to go around. If you're wrong about the young QB waiting in the wings, it's career suicide. And if you're actually good with your rookie contract QB, you're not going to be in a good position to pick his successor.
Again, there are fundamental differences between the NFL and college. The biggest difference is the athletes in the NFL. In college, if you have a QB who runs a 4.5 40, he's one of the most athletic guys on the field. So you can get away with him being a more limited passer because it's very hard for defenses to contain his running. In the NFL, there are too many athletes. This year, when we had Hollins and Floyd on the field, we had two EDGE guys who ran 4.5s with a DT who ran a 4.6. Defenses have the athleticism to contain those sorts of QBs.
So to counter that, you either need a QB who can win from the pocket or a QB who is such a freak athlete that defenses struggle to contain him. Lamar Jackson is an example of that sort of rare athlete at QB. But Lamar is actually a solid passer too. Speaking frankly, there just aren't many guys at QB who have that sort of athleticism. And of those guys, most are very limited passers. It's why defenses caught up to RGIII, Kaepernick, and even Kyler Murray and Cam Newton to some degree. Hell, the Patriots tried what you're proposing with Cam Newton this year. We saw the results.