I'd be surprised if USB-C was a limitation on phone technology even by 2040. The bandwidth and power delivery capacity are way beyond what are needed now. Data transfers from phones are going to increasingly move to wireless in that time frame too, I expect.
The limitation on the viability of USB-C with phones won't be the actual technological viability of the standard with respect to phones. Instead, the problem for USB-C for phones will be if another standard comes out and starts being used by other devices that do need higher bandwidth or power delivery capability. Monitors, storage devices, laptops (etc.) will eventually need more than USB-C can provide, even with future updates to its capacity. When those switch over to something new, that will be when phones (and other devices) will need to consider a new standard too.