@petitgrf Fabien- Regarding duty cycle/TDMA point: If the gateway itself is limited in its ability to transmit, it can't waste much time coordinating slots, etc. I think this applies to any gateway, NB or CSS. NB systems have the advantage of having many more channels, so mitigating collisions is less of a problem.
In the US, with LoRa Symphony Link, we use a 2 second frame header transmission which sets slots by spreading factor. Then nodes randomly select some number of slots, based on QOS, and then choose a random LBT interval. This increases capacity by about 400%.
The reason we still like LoRa over NB for Symphony Link is the downlink link margin is very good. Systems like Sigfox or Weighless-P (I think) rely on standard FSK downlink, which has 20 dB less margin compared to LoRa, which can be >-136dBm on both ends. Implementing a FHSS compliant high gain downlink in a cheap chip is a big advantage of LoRa.
For "city wide" uplink applications, I agree that thousands of narrow band channels will provide a much higher capacity.