I don't think this will ever be able to replace current on-the-ground cellular networks in places where you already have good (4/5/xG) coverage. Bigger cities have thousands of BTSs (base transceiver stations - devices your phone connects to) to achieve acceptable coverage and bandwidth. To have comparable coverage and bandwidth for the whole world you'd need not thousands, but millions of satellites. I don't think that's a realistic proposition.
But, constellation like this will have significant advantages outside metro areas, where you cannot get good coverage right now. Also, for planes, ships and potentially autonomous vehicles, this will be great. Another potential use-case is long-distance, backhaul connections. Speed of light is non-marginally greater in vacuum than over fibre so your packet will get to the other side of the world faster than through cables and glass. This is critically important for certain applications (high-speed trading, for example) where customers will readily play steep prices to shave a couple of milliseconds of packet round-trip time.