Riot, if you can make Doom Bots, why can't you make Hard Bots?
For the record, I almost never play bots except to test a troll build or something.
However, I know many people like League, the characters, the gameplay, but don't like PvP. They tend to get bored with the game once they "outgrow" bots and are pretty much forced to play PvP or quit.
I don't think Hard Bots would have negative impact on any queues to be honest. All the bot queue normally take less than 10 seconds as it is, and not enough people who play PvP would be so much more attracted to harder bots to leave PvP queues en masse.