Unfortunately you don't buy an always open 15 Mbps pipe, you buy "15 Mbps peak speeds", or at least that's how it is worded for me. Which means at peak usage times you expect to get lower than 15 Mbps.
It's hard for customers to buy that when the YouTube and Netflix videos they're watching only need a few hundred Kbps, but they can't get one good stream going on an otherwise unutilized connection. It's not a capacity crunch near the edges of Comcast's network, where lines are still shared in neighborhoods and upgrades cost the most; it's just greed, fattening already healthy profit margins by refusing to increase peering capacity and demanding peers pay more for bits customers are already paying for.
It is worse than that. "15 Mbps peak speeds" is actually "15 Mbps bursts."
I am fine with getting less than my maximum speed; I understand that the connection is shared and that my neighbors' service can interfere with mine. TCP can deal with congestion. What grinds my gears is that ISPs are not content to sell me a maximum speed; they want to oversubscribe, then claim that they are selling burst service, then call it "unlimited super-awesome Internet!! 15Mbps!!" then penalize customers who fail to limit their use of the supposedly unlimited service.
Which means deciding what an acceptable QoS is for those peak times - many of us think they decided too low - I personally think you should be able to get between 1/3 and 2/3rds (ideally around half) of 'peak' speeds on a sustained level at any time for any amount of time.
It's just like the telephone network of yore, no single trunk was ever capacitied to mothers day call traffic - there were reroutes, and more than one path to any large point, so you'd get a lower quality of service, but the traffic would still get thru.
