I see your point. Are you a player who does not also play IRL (in real life) Dominion games? I will not speak for others on this site, but this is my perspective, when people ask for an Undo: Are they asking to correct a mistake where, IRL, that mistake would not have occurred? If so, then I grant the Undo. *Even if* they have received new information.
For example, earlier in the thread someone mentioned "Lost in the Woods," where you can get a boon at the beginning of your turn by discarding one card. They way the online game is set up, it is very easy to accidentally select a card in your hand to play, only to see it discarded for Lost Woods. IRL, that sort of mistake just would not happen. So, I grant that Undo, *even though* the other player has now seen what the boon will be. It's just not that big of a deal for me (although I don't blame other players who would do differently in that situation). And if I granted such an Undo, I would also put a comment in the chat, to the effect of, "No problem to give the Undo. But only one of those per game." And I'd make sure I add a 'smiley face', to keep things light and friendly.
[Note: This has never happened to me. But if the other player was dashing through her turn AND also repeatedly complaining that I was playing way too slow, I suspect that I would be much more likely to deny an Undo request that was due to her playing too quickly.] :-)