You can sort of already do that. Go into the game archive of your game and then go to the analysis. Near the bottom right is an icon of a target with an arrow in it (next to the share icon) and this is Practice vs Computer. You can then finish the game versus the computer for practice if you want.
Replace my opponent with a bot when they resign
Yeah, actually i got a DM from a staff member providing this link explaining how to do that:
https://support.chess.com/article/684-how-can-i-play-the-computer-from-a-custom-position
I guess my suggestion then would be to make it simpler for the user, so for example when an opponent resigns, the modal (pop-up) that appears could have a button labeled "continue vs. a bot". That way the path consists of one click vs. however many the current method requires.
Wouldn't it be cool if when your opponent resigns you could continue the game (against a bot)? Your opponent would be replaced by a bot of a similar rating.
Many times my opponent resigns either out of boredom or because we've reached the endgame and I have more pawns, but I want to continue playing to see how I'm going to use the pawns to win. So with this feature the bot replaces my opponent and I get to keep playing.