User traffic is user traffic, if they're paying for it it should be all treated the same. I'm not saying you don't need to prioritize some traffic with QoS. I'm saying you don't need DPI to run a network.
A network engineer
User traffic is user traffic is whole lot of different kinds of traffic. So there should be QoS, but how? You cannot depend on the IP Differentiated Services Field, so perhaps heuristics to deduce what the traffic is? Thats prone to misidentifying Netflix vs. CDN download thus ruining UX.
No DPI is needed.
If you treat your network's users them all the same, more power to you. Running the network and deciding what goes through it is still out of their control. I don't get why you're against those that do use DPI to run their network more easily, though.