Busy streets, corners, plazas, indoor whatevers, and so on.

Furthermore the end goal is natural convergence with WiFi. The telcos already are pushing Ethernet more and more (backhaul and fronthaul), of course over fibre (because bandwidth requirements, and timing has to be calibrated to microseconds because radio symbols are being encapsulated in Ethernet).

MP-TCP has been recently merged to the kernel, it was already enabled on a lot of phones.

What's missing are ways to allow hosting secure hybrid-cells/APs so telcos are able and willing to offload traffic.