Not only that but your idea in general hurts FOSS communities. It would encourage an atmosphere of “give me money or I won't release bug fixes” (bugs may already been fixed by the dev in private but not committed into git).
If you just looked closer at GSOC you'd see that very often someone appears to develop a feature, gets accepted into GSOC, implements the feature fast and therefore buggy, and after collecting the money leaves without ever being heard of again. A case that comes to mind is KWin’s tiled window management. The code had to be removed for 4.10 because the GSOC developer did not maintain it.