You make several valid points, and I had the same thought about how ironically the dirty air racing on the winged side has more parallels to modern formula cars.
However, the real point I was making is that there is zero cultural connection between winged sprint cars and IndyCar. Nobody rising through the ranks there gives half a thought to IndyCar. Some of them would gladly make the NASCAR leap if it landed in their lap, though those opportunities have largely dried up as compared to 15-20 years ago.
WoO and winged 410 racing is on a trajectory to become more of an end and destination unto itself.
On the other hand, USAC still has a fleeting connection to IndyCar because it so Indiana-centric. The myth and lore of the Speedway looms over everything that happens in that state.
Clauson wanted to do IndyCar (after NASCAR fizzled) because of his ties to the USAC world. Windom wanted it as well, though now he's followed Courtney over to the winged side, because it's their best real chance to make some real money. As soon as they go winged racing, the slight chance they would pursue a restored Indy Lights type ladder feels over.
Many of the younger drivers who pursue USAC as a ladder from midgets through Crown cars would still leap at a developmental IndyCar deal, whether the cars share any real similarities or not. It's just in the water in that part of the country.
Of course, rebuilding that bridge would require at least two or three more thriving Iowa-type oval dates on the schedule. We're still a long way from that.
First, the rear-engines and USAC thing: I know when I found out about that having happened (I'm a young whippersnapper of fourty years old wait what the) the notion that USAC banning them prevented them from sticking with the Indycar ladder generally went right along with it. Obviously it is unknowable, but I think about the parallel development of supermodifieds and I just don't know in the long term if it would have mattered that much. They might occupy that same position in the racing universe that Outlaw Late Models with the giant plexiglass billboard "wings" on the side have today.
Ultimately, aerodynamics became A Big Thing and once the genie was released from the bottle, there was no stopping it. IMO 25 years from now people will talk about the decision of USAC to not go in with winged sprint cars as a colossal folly bigger than any other they committed, and there are a litany of them. It will simply require a sufficient number of people who prefer non-wing cars over wing cars to die out (sorry to say).
If you stop and think about it for a moment, what you say about winged sprint car drivers not having a connection to Indycar is essentially because wing car were banned from USAC. And USAC is, has, and will always have that special relationship with Indycar due to all the stuff that happened between 1956-1978, the geography, etc etc etc. USAC's decision making in that particular instance to protect what they saw as their constituents doomed them. They rejected a product that was basically guaranteed on some level to supplant them, and in time that happened. They might have gotten a couple more good decades out of non-wing cars that way off the USAC name, existing fans, and so on, but like in all things Father Time remains undefeated.
Everything else in the discussion comes back to money. I have tended to look more and more at racing more from my own background in finance, and the more I look, the more things sort of fall into place in ways that maybe don't line up obviously to others. If you look at the Indycar ladder going back to the 80s and 90s, the transition to pay drivers is ultimately led by the loss of having a self-sustaining ladder over costs. NASCAR is full of tube frame chassis at every level that can be built in much more rudimentary shops; Indycar (by necessity) made the transition from tube frames to composite and honeycomb monocoques derived from aerospace tech that cannot be easily duplicated in someone's prefab steel structure backyard shop. NASCAR overwhelmingly raced ovals where its easy to run events from a ticketing, security, and race management perspective: ovals resulted in big crash damage for guys racing composite cars so they preferred road courses where it is much more difficult to run events due to the complexity of all of the preceding things ovals are good for. Bill Tempero all but confirmed that his American Indycar Series was intentionally avoiding superspeedways for that specific reason. It just sort of spiraled that way not because anyone had bad intentions, but because That's The Way Things Are.
Of course, costs in NASCAR are now out of control and drivers aren't hired on merit there anymore either. David Gravel: 2 races in Trucks, 1 in ARCA, said publicly he needs to find sponsorship to put together another part time NASCAR deal of any sort. John Wes Townley: 186 NASCAR national touring series starts and an additional 58 with ARCA because Daddy Runs Zaxby's before he was retired by his ex-wife's boyfriend at close range.