If Lucas had made prequel films that were true to the spirit of Star Wars, they'd be much better received. The prequel trilogy was always meant to be a bit different in tone, but the one he put out was a whole damn different galaxy away in tone and mentality and that should have been avoided (and could easily have been avoided). A bit more effort to fit in with the story of the originals would have been no harm either.
It's also evident that Lucas must have changed some things from his original backstory. Some of the lame things that look like new inventions may actually go way back, but I'd guess there's still plenty stuff that was changed or newly invented. And if some of the crappy things go way back, well that's where he should have applied a bit of editorial stringency and cut the shit. In the OT he kept out some things he'd invented back then that were best left out (such a midichlorians) and it would have been good if he'd done the same with the prequels. As in no Annie builds 3PO (yes that probably does go way back -see the expanding the universe chapter of The Making of Star Wars). The originals showed some judgement, while the prequels didn't. Surely it would not have been that hard to apply some judgement in the making of the prequels?
The originals had honesty, sincerity, genuineness of feeling and imagination. The first prequel had hints of that mixed in with the shit and shallowness but was overall an artificial film that was overly sweetened up and didn't believe in itself. The later two prequels were horrific examples of shallowness, lack of emotional depth, soullessness, artificiality, stilted crap and general bullshit, with terrible judgement. It would have been fucking EASY to avoid making the films like that. Just as it would have been fucking easy to keep the podraces and cartoon characters out of TPM and make the central character less lame and limited.
And lame and limited as lil Annie was in TPM, he was light years worse in AOTC and ROTS. That has to be hands down the most horrific destruction of a great character ever. In ROTJ we meet non-evil Anakin and you strongly get the impression that there's really something to the guy. He could NEVER have been the shallow spiteful wimp Anakin of AOTC and ROTS. Him having something to him was crucial to both ROTJ and the trilogy as a whole. And Vader too could never in a thousand universes have been the Hayden version of Anakin. The total break between the prequel and OT versions of Anakin/Vader is a massive break that could easily have been avoided. Clearly Lucas entirely reenvisioned the character, with seemingly no concern for what the OT had established. That reenvisioning makes it entirely impossible to take the two trilogies together as the one big six-part movie Lucas wants us to them as.
Also I've read things about the prequels that sound to me like Lucas pretty much deliberately making them bad movies. For example, in an interview he talks about how he wants to make the AOTC romance in such a way that men and "cynical" (read "intelligent", I think) people won't like it. Or something like that. There's also a quote from McCallum about how they knew as far back as 1990 that the first two prequels were going to be "hard" (I think that was his word, maybe it was "difficult") with anybody who had any connection to the original films (by which I think he meant anybody who was a fan of them) and anybody over 18. Or something like that. It sounds to me like "We set out to make films that would be at odds with the original trilogy and which we knew people would hate". And then there's this thing about making the prequels like old serials, which seems to me (after reading Kaminski's book) like it was taken as a license to make shallow films that lacked emotional depth. There's also the example of Palpatine's ROTS makeup being done to echo old horror movies. Jeez, why not make it to echo ROTJ instead, for god's sake? That makeup was horrible and now it seems to me that pretension about homaging old films was behind the awfulness of it. It seems to like Lucas had a whole collection of reasons for doing what I'd call deliberately going out of his to make the prequels bad movies. It didn't need to be that way.
So no, it was in no way inevitable that the prequels be awful movies the way they were. That was something that took effort.