DSL proponents were telling me this in teh late '90s, that my 512kbps Cable would never scale because it was "shared". My 30Mbps service Speedtest's at 31-32 all day, every day. DSL lines don't go directly to a backbone either - everything is shared at some point. If you experience slowdowns, it's poor engineering, which can happen with any medium. And it will always happen when the network is carrying the heaviest load There's no inherent' problem with cable.

In aggregate, cable is becoming the dominant delivery vehicle. But if your cable provider can't fix it, it's certainly possible that it's poorly engineered or incorrectly installed. If DSL is better for you , go back to DSL.