I can´t help but I think you are a bit disingenuous here. You are right that as the interim moves out towards the final there is more power in the interim. You are also right that the chances of failed interim and a successful final are getting lower. However, the nominal alpha for the final is always above 0.04.
Not at all disingenuous - It all bears on investment approach. Before the recent rejiggering of interim and trial size I had thought about going long after the failed interim.
Now that approach doesn't have legs - because of the aforementioned effect plus the overall reduced power of the trial. C'est la vie.