The funny thing is I quite enjoyed the whole FFXIII-saga, with LR being my favorite and the most fun I had with a JRPG in a long time. BUT I deliberately lowered my expectations to basically zero so I could enjoy them and that is not what I should have to do with a (mainline) Final Fantasy game. When I played FFXIII-2 and LR I had absolutly no emotional attachement to anything happening story-wise and I just played them for the gameplay and the amazing music. The writing was atrocious with the most unnecessarily convoluted, retconning and incoherent stories. To be fair, I enjoyed some of the characters' background stories, like Kaius/Yeul's or Noel's, which had a somehow pretty unusual melancholy to them if you break them down to the basic human elements and ignore the convoluted, eso-metaphysical non-sense around it. Btw, LR has the best battle-system in the series, IMO.
I'm not a blind FFXIII hater, but nobody can deny that it had its fair share of problems and big flaws game design-wise.
The developers basically admitted they had no vision for years during development and rushed it in the end. I specifically said "user reception" and FFXIII has the lowest of the mainline FFs on Metacritic (AFAIK - only checked the modern FFs since FFVII). On the other hand, maybe I shouldn't use Metacritic as an argument, anyway. Some of my recent faves have low scores (LR and Deadly Premonition). So sorry for that.
Thing is, it's still very polarizing and - if warranted or not - has tarnished the brand in the eyes of young, modern gamers whose only exposure to the FF series is hearing how FFXIII sucks on all comment sections and message boards whenever FF is mentioned. Many of those youths had no new mainline (singleplayer) FF since FFXIII to play themselves yet (it'll be over 6 years when FFXV comes out!) and most likely don't even know that all mainline games are totally independent story- and gameplay-wise, so the whole franchise has this negative connotation for them.
Who knows, maybe one really good, well-received FF (from press and users) is enough to get many new and old fans on board. We'll see next year when FFXV releases!
Oh and regarding your statement about FFXV not being "traditional": gameplay alone is not what makes FF. What people remember and cherish the most about their favorite FFs is the unique art-style and unusual mix of settings, the epic world-spanning plots, the dramatic stories, the beautiful music etc. FFXV seems to have pretty much all of those elements too
if it hopefully delivers on those premises. The only thing that's really "non-traditional" is the battle-system, anything else is 100% FF in my eyes, which is hard to define anyway, as outside of certain superficial tropes (Chocobos, Cid, fanfares etc.) the only constant of the series is "change and reinvention".