Perhaps we can price to regulate how much traffic comes in, without reserving anything. No buffers in the network for delay-sensitive traffic.
Users buffer at the endpoints and choose a rate, the network guarantees it.
Users can renegotiate. See the paper on RCBR in Sigcomm '95 for details. You will always have users who want performance bounds. How much do you lose if you serve them CBR, and reserve a CBR rate for them.