Randomusername. I definitely see it as very similar in the sense of "I now realize that I can't win after executing my INTENDED plan".
To anyone still following this, let me boil this down to a broader family of question. This is now not about justifying my previous game, but about how undo's should be handled late game.
Let's assume that in a different hypothetical hand, Villain is on his last turn and if he doesn't end the game, Hero will. Villain has different ways he can play out his last turn. One way would give him a 5% chance of victory, another 20%, another 50%, another 80%, and another immediate victory.
Villain, unaware currently of other plans, chooses to go with the 5% plan. After executing he realizes that this leaves him with roughly a 5% chance of victory.
He reconsiders and realizes there was a plan that would give him a better opportunity, and requests undo to go with the 20% plan. After executing he realizes that this leaves him with roughly a 20% chance of victory.
He reconsiders and realizes there was a plan that would give him a better opportunity, and requests undo to go with the 50% plan. After executing he realizes that this leaves him with roughly a 50% chance of victory.
He reconsiders and realizes there was a plan that would give him a better opportunity, and requests undo to go with the 80% plan. After executing he realizes that this leaves him with roughly a 80% chance of victory.
He reconsiders and realizes there was a plan that would give allow him to win immediately. He requests undo and goes on to win outright.
I assume most would agree that something is wrong with villain doing this (but maybe I'm wrong in that assumption). At what point does it become wrong? Should this maybe just be granted once (although that doesn't really seem justifiable)? Is this scenario different if the probabilities changed (such as changing 100% win to 70% win as the upper limit, or maybe even something low like 30%)?
Most people who have responded to this thread make it sound like the following information does not matter: villain only considering other options after he played out his entire buy phase as INTENDED to then realize that the victory counters looks very unfortunate for him winning.
If no new information was revealed, villain/opponent can try whatever he/she wants (5%, 10%, 20% backtrack, 30% whatever), and I will grant the undo. Again, I want to emphasize because it seems you are conflicted about the definition of fair undo (which is subjective anyway, and a made up term according to consensus here on this platform), if no new information is revealed, undo grants are considered fair and should be granted
Buying duchy and wolf den kicking in, is not acceptable, because you should have kept track of your buys..another example, buying gold thinking palace would give you 3vp but realizing that you should have bought silver instead is a kind of undo I wouldnt grant, because again, you should have kept track of those buys. In the last two cases, new information was revealed which wouldnt warrant an undo, and i have seen top players not even asking for an undo in those cases, because they realize it is not fair.
Let me give you another example, I was playing with a very good player who tried to do a 3-pile and would have won, but the game involved page, and page was one of those 3-piles, but by mistake he ended up returning pages in play so 3-pile case was not there anymore (auto exchange is tricky, very tricky), and he ended his turn. Now I could have won, but i knew this was a genuine mistake, so I immediately granted him the undo. Your opponent, similarly, realized he made a mistake with buys and he wanted to fix it. "Enhance" is a new card, we are all learning these new menagerie cards, and the interface is not super intuitive for some things, at least for people who have not spent hours on it. Is it too wrong to expect some slack? Trust me, if you continue to deny undos towards the end of a game due to one made up reason or the other, but those undos were fair ask, you will get blacklisted, and soon you wont have good players to play with.
I feel i repeated myself a lot there. Sorry