Does any sect claim that only Shakyamuni will ever achieve great nirvana ARe there any schools which say that Shakyamuni the historical Buddha, alone, will ever be a Buddha?

I understand that some might identify him fully with the dharmakaya, but isn't it completely canonical that there is a Buddha vehicle for others, at some point in time?
No, there are no schools that claim this. All the schools accept that there were previous Buddhas before Buddha Shakyamuni and that there will be Buddhas after him as well.