Scalability and quality of service: a trade-off?

During the last decade, two big efforts on Internet quality of service were made. The first, IntServ, promises precise per-flow service provisioning but never really made it as a commercial end-user product, which was mainly accredited to its lack of scalability. Its successor, DiffServ, is more scalable at the cost of coarser service granularity - which may be the reason why it is not yet commercially available to end users either. This leaves us with the question: is there a fundamental trade-off between QoS and scalability? A trade-off that, in the long run, could prevent deployment of QoS for end users altogether?.