Absolutely. The saga was basically a closed arc.
I think that TFA/TLJ really works as a closed arc (as a friend and I were discussing yesterday, I think TLJ would work much better as a stand-alone film, with the stand-alone and un-followed-up ending being all that's said), or else as the pilot for a very LONG story about the Resistance rebuilding and a prolonged conflict. What it doesn't readily set up for is resolving the entire story in one film (although arguably, the same could be said of the ending of ESB). I think the gambit of bringing in Palpatine to try to make this *feel* like it was all a part of a bigger saga is the best way to both make the sequels feel tacked-on, and bring about a resolution to the trilogy that feels suitably grand and relevant while still working from the starting-point they've been given.
Not that I think it's necessarily a good
way of accomplishing either, or will succeed in doing so- but it kind of feels like if anything can solve all the problems at once, this long-shot is the only Hail Mary Pass they've got.
But, trying to pretend it is necessary or the logical way things had to conclude would be disingenuous.
That said, reading between the lines, "I don’t know many books where the last few chapters have nothing to do with those that have come before." could actually fit in the broader context- in that the handoff between the PT and OT involved a contiguous scenario and shared characters all the way through; the hand off from OT to ST involves a non-contiguous situation (whose genesis has been largely explained by non-film materials), and the shared characters have mostly either been killed off or (in the tragic case of Carrie) are unavailable.
Now, the multigenerational epic model does not preclude a model in which, in essence, the final chapter has only
the most recent generation left alive, and are running through a plot that is not an outgrowth of the primary plot of the book, but a separate tangent that came in when they did... but, presumably, it is more common for the ending, even if characters from the early generations do not return, to have a situation or storyline that has organically developed from what came before. So, in that case, I could take JJ's statement 'where the last few chapters have nothing to do with those that have come before' to mean less "Therefore we must bring back a character from the earlier generations in order to keep it relevant," but rather "Therefore we must tie the current scenario more concretely into the overall narrative so that the situation the last chapter ends on has some relevance to the whole- and a returning villain who orchestrates grand, long-term schemes happens, incidentally, to be the best way of doing that." So it's less that Palpatine is that 'something to do with what has come before,' but that he is the vehicle
through which something to do with what came before can be brought in.