I think a lot of the allure that draws the public to Unlimited air racing is the sound of the "muscle planes" with their big throaty radials and screaming V-12s. I'm not saying there isn't room for a turboprop class, but I can't see it replacing the recips in Unlimited racing. Ultimately, it's all about the race promoters and vendors making money by selling tickets to the public. Variety is good, but I think in this case, they need to "dance with the one that brung 'em". I personally would love to see more experimentation and development take place, but not at the expense of The Fun Stuff. It's true that there's a finite supply of vintage recip engines available, and racing does take its toll on them. But how long will it be before some innovator like Jack Roush comes out with complete repro Merlins? He's already making heads and other parts. Can new engine blocks and cylinder banks be very far behind? As for airframes, that's an area that's wide open for experimentation, even in the existing recip-powered Unlimited class. And it's already happening to an extent. 20 years ago, who'd have thunk that someone would take a lowly little "red-headed stepchild" Yak trainer and turn it into a screaming Unlimited racer? The Pond racer was a bold step. It wasn't truly successful on any level, but at least someone tried.
You mentioned NASCAR and their pushrod V-8s (which, by the way, crank out 700+ HP literally all day with very high reliability). Yes, a lot of people, like my wife, think it's boring. Millions and millions of fans will disagree. What other auto racing event in the world, with the exception of the Indy 500, draws 250,000 fans a week to the track and has millions more watching on TV at home? NASCAR is popular because of what it is, and what it isn't. What would happen if NASCAR changed the rules and switched from those big, mean V-8s and forced everyone to use little four-bangers? There is certainly plenty of technology available to make a 'banger crank out some serious power, no argument there. But would their buzzy exhaust note and whistling turbocharger draw the crowds like the big ol' rumbling V-8s do? I seriously doubt it. And have you ever driven a Sprint Cup stock car on a NASCAR superspeedway? I have, and it would definitely not be the same without that roaring V-8 up front!
As with any controversial topic... YMMV.
We used to live in Oxfordshire in the UK and were blessed with the overflow from the (UK) F1 GP there at Silverstone. We've also been in the northern side of Melbourne and heard the whine from the (Aus) F1 GP from the other side of the city...