Whenever their performance drops towards part of the internet, their value proposition gets lower. You can view the value chain as
(residential customer) <-> Comcast <-> Level 3 <-> (hosting customer)
Both Comcast and Level 3 have customers, in opposite ends of the currently centralized internet. The status quo is that each provider charges to the customers on their end. Comcast (and each of the other non-net-neutral ISPs) are eyeing the revenue stream on the other end of the pipeline. This revenue stream has historically not been owned by the last-mile ISP.Level 3 and equivalents, by cutting off non-performant interconnects will hurt their own customers, of course. However, they are just speeding up the same effect being applied softly over time, by Comcast and equivalents. A softly failing interconnect moves revenue over to last mile ISPs, thus permanently lowering Level 3's value proposition. By failing the interconnect sharply, instead of softly, they actually improve the steady state of the system, in general (good side effect), and in particular (the real objective).