As far as I know hold sliders are not allowed for ranking. They were allowed until early 2010 or a bit earlier (an era when the maps were... ugh...)
Well, it may be okay for osu!stream/droid users. But almost all players are in PC version so it would be senseless to hold the mouse in a game where you got to move mouse/pen constantly.
And if overused, they would make VERY boring maps.
Not to talk about CTB, it would make maps absurdly easy (Do you know how stressing is to stand still and then being attacked by jumps?). And people overusing HT in Taiko, etc.
So, NO. I don't agree. UNLESS there were a fifth game mode that would be played with multitouch screens, but as I said, we mostly play in PC and a considerable percentage of users doesn't have a touchscreen, plus, it would be arm-tiring to hold your fingers into a 32-inch monitor.