I now agree with this, as a Love Live! School Idol Festival player (that game is similar to mania 9K, for mobile/iOs) and it works great
Though not all maps will be properly playable
mania would need a large UI revamp to get anywhere close to LLSIF, though.
Either way, as osu! is a Windows game, I could see this working (as most touchscreen PCs have at least 11' screens). About the input lag, the offset can be calibrated (and a visual offset feature, where the notes appear earlier than judged) could be implemented.