Problems with lots of small planes:
* They fly like a roller-coaster (4 seaters are crazy scary for many people).
* More crashes - bad "optics"
* Number of pilots required
They do have the potential to have more direct routes, and very short hops, with less security overheads. Maybe run more regularly, but not much of an advantage because most people want to travel at particular times of the day (same problem with buses).