I'd say the new bowser car set is a perfect example of how course builder Mario Karts sets would work. It's incredibly well designed, and the scan code putting a steering wheel on Mario's chest and allowing him to honk the horn by the pull of a lever, while launching bombs behind the car is genius. I wouldn't be surprised if we got more sets like it. The code in bowser's car has no indication on it of being exclusively for him, so it can be reused.
I don't that this line deserves to die and be replaced fully just because AFOLS are disappointed by it. The fact it's been going this long proves it must be very popular with it's intended audience. I'd say it's quite harsh to say that it's unimaginative and sacrificing quality and excellence, because that's just not true. Deciding to not make figures like every other theme was a bold choice, and understandably some don't like it. The technology that makes the characters work is very impressive, to me it's one if the most interesting developments that LEGO have made in a long time, and one of the only examples of them tying a digital feature to sets being very successful. The designers have listened to feedback on the design of the sets too. I agree that quite a lot of them don't display well, it's kind of the idea that you take the sections apart and rearrange them, but recently, the builds have been a lot more solid. I think that the 3 airship sets and Dry Bowser's castle are good examples.
As a platforming game, Mario unfortunately is not easy to adapt to playsets. Scanning codes with electronic Mario to interact with blocks, enemies, and other hazards was a great way to achieve it. Figure sets trying to emulate the platforming aspect just wouldn't work as well. I think they actually would be unimaginative and not as fun for kids to play with. Mario Kart would work as minifigure sets because you can play with the cars and move them around, but the sets would then need to be designed around cars driving through them, presumably with track pieces and other gimmicks, so they wouldn't display well either.
At the end of the day, there is no way LEGO didn't explore minifigures when they obtained the license but there's obviously a reason why they didn't go that route, so we got the course builder system instead. I think the 18+ sets are the perfect place for LEGO to give us minifigures and great display models, like Peach's castle, while keeping the aspect of the theme that you can actually play with that's true to the series' nature alive.