Suppose the majority of ISPs was to move to a direction were some significant portion of internet services was "shaped" or throttled in some way that was imposing real disadvantages or inconveniences for the user: wouldn't that open a seizable opportunity for a new player to offer "real" unrestricted connectivity?

I believe it if someone walks us through the incentive structure here step by step, or explains the natural-monopoly nature of this market or something. But just by implying it or pretending this to obvious I'm not convinced.