I fail to see why the game needs a sequel just because of its traversal mechanics.
SO traverse mechanics were in part inspired by their work in R&C but increased by 1000%, while Spiderman then took everything they learned in SO and downscaled it to fit the swing mechanics - using what they learned during jump off points and connecting traversing mechanics for a seemingly flawless flow.
Case in point:
SO does not deserve a sequel, but it is, to me, an overlooked important moment in the company's history as that game helped transcend them into creating a better Spiderman game.
It should be remembered, but it is not of any importance to bring it back for a sequel - other than as an excuse for a game to further experiment even crazier traversal mechanics for future games.
But you say this like if traversal was just secondary, just a detail, but it's not the case, traversal is everything in a game like this, it's part of what made it so amazing, part of the fun.
I could play Sunset Overdrive for hours and hours just free roaming around, enjoying the fun gameplay, trying to pull off some crazy stuff, like how I would free skate in a tony hawk game or something else in that genre.
And like you said, they "downscaled" it for spider-man, and I mean it was kind of necessary to fit with Spider-Man, I'm not saying it as a negative, but I don't find Spider-Man as fun as sunset overdrive gameplay wise, especially if you play Sunset Overdrive with keyboard and mouse, the potential is amazing and you can really do crazy and super satisfying stuff, since you have a lot more freedom / speed for turning around, aiming and such.
Spider-Man, while being super great for portraying spider-man, is slower and not as arcade-ish as Sunset Overdrive, it's different and not exactly comparable, but all of this to say that no, Sunset Overdrive didn't just serve as leading to Spider-Man, it's a different proposition that might fill different needs. I really love crazy fast paced stuff, scoring and such, so Sunset Overdrive was really paradise for me, Spider-Man, while being amazing, doesn't deliver the same exact old school but mastered arcade feeling.
And it's not just the traversal itself anyway, it's a big part of it since it's what led to this amazing gameplay, but I absolutely loved the art style, the atmosphere, the humor (apparently a lot of people here hated it, but it worked on me), and most importantly, the game / mission design:
Having all these short but fun objectives all around was super satisfying for me, it felt just like doing challenges in a Tony Hawk game (especially project 8 which has similar environmental challenges and stuff), or the ones in SSX 3. You have a big open city / world, and instead of it being filled with boring side quests like any other modern game in a ubisoft style, you had short, satisfying and fun challenges everywhere focused on scoring and getting better, it was fun and addictive, and helped the game not feeling tedious and overwhelming unlike most other open worlds today.
Fun was really the core of this game, it really is like an old school, ps1-ps2 era arcade game, but pushed to the extreme with modern technology / possibilities, in a AAA / open world scale.
I get that if some people only played it for the story, without trying to push the gameplay nor doing in crazy / fun stuff, the game might not be that great, and spider-man would definitely be a better offer for this since it rewards the player more easily with "impressive" stuff for free. But if you really are into arcade style gameplay, if you really like enjoying yourself with a given gameplay and finding your own ways to have fun in the city, if you like scoring and such, Sunset Overdrive is an amazing game, and it definitely can't be replaced by something like Spider-Man (as great as it is, again, I'm not talking negatively about Spider-Man, but they just are 2 very different propositions.)
The world in Overdrive is like technicolor cyberpunk it's so fun to roam around in. How about some flying cars you can surf on? Yes please. Make a sequel. 
