Rules and pricing needs to be simple and consistent. Even if it was possible to implement QoS to broadband, it would be awfully confusing for both the consumer and the telco. Why should someone in a busy area be capped when they reach the limit, while someone somewhere else can keep going? It might make sense from a technical perspective, but not at a consumer level.
If the telcos are not price gouging, there shouldn't be a problem with data caps. Stop being a cheapskate!