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.
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.
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.