They're not competing for bandwidth with anyone else yet. Wait til the service is open to everyone.

They're limiting the number of CPEs per cell to a maximum number of sites, and volume of traffic, such that the service won't degrade to a viasat/hughesnet like terrible consumer experience. I can't say how I know, but some very experienced and talented network engineers are designing the network topology and oversubscription/contention ratio based out of their Redmond, WA offices.

In rural areas, contention should stay low enough that the speeds will continue to be at least decent. And of course the latency is much lower than geosynchronous satellites, independent of bandwidth.

OneWeb is launching new sats I think in leo