I will start off with the fact that I do not care about any books. The films should be self sufficient.
Of course the war could probably not last continually for 30 years. Nevertheless, it is safe to assume that Empire did not cease to exist at the end of ROTJ.
Even if we somehow play stupid and assume that practically no development was made on both side in 30 years, there are many new ships on both sides in ROTJ that are not in TFA.
It just a dirty rehash. That is the reality.