Fair enough. However this is a video game sales tracking site and typically the best way to track a sequel/reboot's sales data is it's history. With that said, this GOW is very different as you mention. This leaves us with little to compare it to. So how is one meant to predict the sales of something if it has no comparable data? The only semi reliable way is to review it's history.
A change in the directional tone of a game, or its quality, can be very significant. I was never interested in the God of War series until now. I'm definitely playing this one. There's also how they are marketed. The west weren't really very interested in the Yakuza series, until Yakuza 0, which wasn't really any different from the rest, but it had a lot of promotional awareness, like the meme of the real estate manager Nugget the chicken, which started off as a random tweet, and now Sega are printing Nugget business cards. These things got more reviewers interested in trying the series, which lead to it getting more critical acclaim, etc.
Persona 5 isn't particularly different than P3 and P4, but it was promoted with an emphasis on its style, and this seems to have worked out well as it hit 2 million in November 2017.
These are factors are hard to quantify though, so people will have to go with their gut feeling at times.
When it comes to Spiderman, one comment I see often is that this new one reminds them of Spiderman 2 for PS2/XB. And that is a good thing because that was a good game, and the last time I was interested in a Spiderman game, until now. Spiderman as an I.P. can be interesting, depending on what you do with it. And Insomniac's take on it is looking a lot more interesting than any previous entry in the series in recent times. This shows in trailers, so I would not be surprised if this one manages to outperform many of the previous Spiderman games.
If the previous Spiderman games were developed by the same developer, and were in the same vein as this one, then I would put more weight on comparing to previous sales. Right now I think that's not as big of a factor as it could have been.