I don't see the point in the suggestion to be honest. I think it is trying to solve the effects of a greater problem, when in reality the cause is what we need to solve. And the cause of the problem, as you mentioned, is forfeiting (i.e. lining up less than 3 players). I think a better suggestion would simply be to allow teams to have whatever number of players they want, and line up as few players as they want, but simply make it that lucky fans will be used in the game.
So if someone lines up 2 players in a game, then 3 lucky fans will be used.
And if someone doesn't line up any players, then 5 lucky fans will be used.
This will effectively make having 5 players on a bot team irrelevant I think.