You add a barrier here. You think that your solution means that AI is reduced, but you also reduce real humans. I noticed this with other parts too, such as "you need to verify your identity before you can post to the ruby issue tracker". I can do so, but I need my tablet and this takes me more time than before, so I stopped using the ruby issue tracker altogether. (It's not the only reason, but adding barriers really makes me invest my time elsewhere - more likely to do so at the least.)
You always need to consider all trade-offs. Charging money means you will also offset real humans at the same time. And it's not solely about the cost; it is simply a hassle to want to do so. For similar reasons I also rarely register at a phpbb forum - I need to store the password to not forget it etc... so more hassle. Using a password manager is also more of a hassle.