This.

I have 3 currently idle 12th scales sitting on the shelf. I've always had a love & fascination for this class since I first read about it in an RC car magazine in the early 90s. To this day I love the simplicity of the cars, I love their speed and looks and their performance. I tried getting into it several times over the last 10 years or so. The last few times with a lot of research and practice and some trial and error I managed to get the car setup right that it drove easily, i.e. not erratic, not tweaked, right diameter tires, right amount of traction compound and all the little things that need to be done correctly to really get the best out of the little car. It worked really well. Yet....I never enjoyed driving it as much as my touring car. The combination of 4wd, full independent suspension and rubber tires just makes it sooooo much easier to drive and also to setup (in my opinion). It's much more forgiving to errors in both driving and setup.
(Of the aforementioned three aspects the 4wd obviously has the biggest positive effect in making a car easy to drive, it's just physics.)

To be really fast, the touring car setup of course needs to be spot on, as any racing car. But you can live with a less than perfect setup in a touring car much more so than in a 12th scale. A less than perfectly setup 12th scale is a nightmare to drive (in direct comparison to a less than perfectly setup TC). And even if they're both setup perfectly, for my humble skills the touring car remains and always will be the easier car to drive. People may say that I have not really mastered setting up the 12th scale correctly and that if I had, it would drive just the same as a touring car. Those people may be right or wrong, but even if that were true, then the effort just simply isn't worth it.

I can only guess, but I think that I'm not alone with this experience. I'm sure many people have tried 12th scale but not many have stayed, mostly because they are just harder to setup and drive than most (or all?) other classes out there.

So, where does this leave me in this rubber tire debate for 12th scales? I would be interested in trying a rubber tire on a 12th scale. But I also don't believe that it will change the reason why 12th scale is not and never will be a class for the masses: it's just too difficult to appeal to the masses, because it's 2wd, it's super fast and the amount of movement on the few moving parts is so small and therefore requires extremely exact work. If a rubber tire were actually able to make the 12th scale drive more easily (because it provided less traction and therefore required less speed) then I think 12th scale might grow, slightly, but not massively. I don't see physics working out that way. 2wd is always harder to drive than 4wd, no matter what tire, no matter if offroad or onroad.
Too little traction is of course also no good, look at F1 outdoors on a dusty track...that's just about the worst RC experience you can have in RC :-)

(Side note: I am aware of course that 10th scale 2wd offroad is more popular than 10th scale 4wd offroad, although there too, 2wds are harder to drive. I attribute this to the 2wds being less hassle to build, maintain and repair, because they don't break as easily and have less moving parts. But the difference in difficulty is much smaller than between 12th scale and TC.)

Nevertheless...I think it's still worth giving it a shot with the rubber tire on 12th scales, we might all learn something and if we stop trying new things we stop progressing. And I don't think anyone claimed that the rubber tire is being tested with the goal to make 12th scale a class for the masses.