The #1 HN user has 10x as much karma as the #10 user. The #10 user has 10x more karma than the 100th user. The #100 user has 10x more than the 1000th, etc. Essentially the vast majority of highly upvoted submissions come from a tiny group of already trusted people, so by fixing the problem for just 1% of users you are actually fixing the problem for ~85% of content. It's almost impossible for a bot to make it into this group, and if they do then it's trivially easy to ban them.
Your fix is self perpetuating in a bad way. If only the top 100 users can get on the front page, only the top 100 users are going to get upvotes. It's impossible for real people to make it into this group.
