There is one huge difference between XV and XIII to me: XV brought in a surprising amount of newer players compared to XIII. I haven't seen this many new people jump on board with a new FF game before.
My point with this is that a lot of these newer players aren't aware of what past FF games were comprised of. They didn't know that the games had engaging stories, full character development, strategical combat, and a huge amount of fantasy elements including unique settings, summons and magics. Therefore they don't see its flaws or its lacking components nearly as much as an OG Final Fantasy fan would.
On the surface this sounds great. I think every gamer should come into a new game neutral and open-minded. But this is a series in its fifteenth entry in 30 years, so there has to be some sort of standard and consistency. You have to consider the legacy of Final Fantasy each time a new game comes out, and how it retains core elements from the series while creating something new.
Ultimately though, I try to base my opinion on fact more than anything. I do think a lot of the numbers point to XV being worse received than XIII -- at least by critics.