All the modifications are already done, its just the OpenWRT team that maybe has decided not to include it. They temporarily dropped support for the AR750s already, but users have been pushing fixes for it to be included again. You can read about it here, and the progress:
All the sources are here:
And here is the AR750s NAND:
Also when i said 3 years, its not a hard limit. The AR750s is one of the most popular routers that GL sells. As with all open source projects, once support is over, fixes will still trickle in from the community and if you are a good customer and ask nicely 