oxygenchair I recommend reading the GrapheneOS changelog to potentially refine the question.
I believe most of the time the kernels are very similar across devices, except for device drivers -- but sometimes the kernels differ. New Android versions have been released to old hardware (see the changelog). But if it were to become infeasible for some device then it might not happen.
Overall, it's easier to make observations about the past than to issue prophecies about what the GrapheneOS developers will be able to do in future years.
Please note that I do not speak for the GrapheneOS project.