Penske has been trying to run a fine line with the introduction of the hybrids because he doesn't have alternatives without massive outlay of capital. He knows that money wouldn't be coming back either, which is why he isn't spending it. If Honda leaves, all the events they sponsor (Long Beach, Mid-Ohio, Toronto, among others) are also on the potential chopping block along with lots of team support and thus teams. The junkyard formula of the 50s and 60s isn't a viable solution now because carbon fiber tubs are too expensive to maintain and operate. The series and speedway likely gets sold at that point and like you said, the series is gone at that point.
But here's the thing: Does anyone here think NASCAR operates an open wheel racing series? No. Does anyone think Liberty Media wanted to run a lot of ovals when acquiring Indycar? No. The reason why Tony George came to Roger Penske to act as a caretaker for the race and series is because everyone else wanted to demolish what the Indy 500 was and make it something else, whether it be a Cup race or a Goodwood style event. If he had someone else who wanted to operate a racing series more akin to the IRL, he'd have sold it to them. That person doesn't exist.
You might argue that what Roger has done doomed Indycar. Indycar was doomed without Roger. Roger at least gave it a shot and gave it some runway to try and pull a safe landing and perhaps gain ground again and find an audience. If 2022-2025 winds up being like the Peugeot/Audi era of Le Mans and it goes to a dark, boring place for awhile (or forever), at least you got to enjoy that. In the meantime, RTA and NASCAR doesn't have a working deal past 2025 and the sprint car series Kyle Larson is part owner of is instituting charters. Things will get more interesting before they get boring.