You're right about osu! Although it's probably one of the few competitive games where there's no gameplay interaction between players - if another player is cheating, it hurts the overall competitiveness, of course, but it doesn't directly affect your gameplay experience.

It's not like playing a shooter where someone has an aimbot and wallhacks, or a racing game where someone can ram you off the track without slowing themselves down - those things directly ruin your gameplay experience as well as obviously hurting the competitive integrity. I don't think those kinds of games would work at all if they were open-source and without anti-cheat unless there was strict moderation and likely whitelisting in place for servers.