The differences between the technology of Uncharted 3 to 4 can be clear traced and identified; also can be identified are the carried-over aspects of the reused engine from Uncharted 1-3 and The Last of Us (probably even some Jak code is in the Naughty Dog Engine Jenga tower.)
Same as differences between the technology of Spider-Man 1 to 2 can be traced and identified (reference the Digital Foundry we're on this thread to talk about, which points out several tech additions that SM2 has over even the previous remastered SM PS5 releases, subtle or rough as they may be in comparison to expectations) and of course also can be identified are the carried-over aspects of the previous Spider-Mans and other games of this and the past generation.
If "next-gen" is simply defined by whether you feel it's next-gen or not, then that's a very loose and per-person definition. If it's about platform exclusivity, you don't agree that SM2 qualifies as "next-gen" so that doesn't work either. If it's a matter of the engine being totally rewritten, many of the games held up as "next-gen" (Uncharted 4, Ryse, RDR2) are built on previous engines; every once in a while you get a massive rebuild like Decima, but it's not that uncommon... UE5 is the "next-gen" tech we're all pointing to as the future, yet it is an overhaul of UE4 and is scalable down to mobile with even fallbacks from the outstanding features of Nanite and Lumen. And if "next-gen" comes from its technological advancements, you can advance technology without changing the hardware, and you can bold advanced technology on top of existing tech to make use of the new hardware. (Is R&C: Rift Apart next-gen? It's doing things fairly unimaginable on PS4, but it's doing it with many of the same general technology systems used to make Spider-Man 1 on PS4 and Spider-Man 2 on PS5.)
You're LOLfacing at me as I describe your definition of "next-gen" as subjective, but even your example goes along with what JaksGhost said that this discussion is "a bunch of Gaffer are still arguing about what next-gen means to them without clearly and concisely SAYING what that exactly is," rather than providing an agreed-upon definition with tangible examples for the conversation points, or reasons why next-gen tangible examples given in the product we're talking about 'don't count'.
If people don't agree that extensive RT use and fast-traversal and water deformation in Spider-Man 2 still doesn't add up to expectations, okay, but that's what this game is adding. It's doing next-gen stuff. It's also doing a lot of old-ways stuff. Same as a wide majority of games we've all enjoyed as "next-gen" entertainment on new boxes throughout the years.