A friendly fork generally has a natural limit on how many commits there will be on top. Personally I've done this with ~200 feature commits and it's not a big problem (as long as you use incremental rebase of course).
Of course if you're planning a hard fork, merges may be unavoidable. But I've seen too many Franken-linux-kernels which were forked from 4.x with periodic merges whose correctness is impossible to verify. Inconsistencies eventually build up with each merge.
