Switching to AI is what's going to happen though! After a period however long in which it works like it does now. Stef said it sounded good and then I said it was the move and your argument sure hasn't swayed me.
As far as switching to AI... Sounds like a lot more work to implement. And it's still going to foster the bad form of resigning a few seconds before the game is over. If I was about to buy the last Province, it doesn't really make a difference if I do it against a bot. I'll still feel like I was subjected to a rage quit or at least an annoyed gesture. Switching to bot makes more sense if someone resigns earlier in the game. But of course, since the bot will probably be terrible at playing the other player's deck, it's not really going to tell me much about how the game would have played out.
If I can offer my 2 cents, I think it would be nice to have the option to end the game immediately or replace the player with an AI. I'm not sure if that's what you are assuming or whether you expect that the game will be forced to be played to the end, except with an AI replacing one of the players.
You can't force anyone to finish a game
And yes, I would like that option.
Furthermore, you should be able in your personal settings to select
1. Always replace resigning player with AI
2. Always ask whether to replace resigning player with AI
3. Never replace resigning player with AI
Personally, when my opponent resigns, I just am glad for the win and move on.
This of course is for 2p. for 3+, replacing resigning players should be automatic until there is only one human left at the table at which point they can decide whether to take the win or continue playing.