The only way that F1 will be attractive to manufacturers starting in 2026 is if the ICE component of the PU regulations is either marginal or non-existent, or if the formula is totally focussed on cost containment, so that only a modest investment ($50-$75m) is necessary for a competitive PU to be designed and supplied, thus allowing an auto manufacturer to justify the spending as a marketing exercise, not an R&D exercise.
(Prediction: I expect Honda to enter Formula E in 2022 or 2023).
With hindsight, the failure to assure a competitive private PU supply was both an indictment of the PU regulations, and a failure on the part of F1 to assure the existence of a PU not tied to a major manufacturer. As we have seen once more, manufacturers come and go, and when they suddenly up and leave, the results can be very disruptive.
The real irony in all of the events is that Ferrari may have nudged Honda towards their decision to leave, by implementing an ingenious but dubiously legal ICE solution that forced Mercedes to throw a massive amount of resource at their PU program in 2019.That ensured that Mercedes once again had the most powerful PU in 2020, and then, with the Covid freeze, Honda found themselves spending hundreds of millions of dollars only to be locked into having the second-best power unit.