I think your argument is quite solid, and even the estimate for G, as detailed here, is not crazy. What will happen though is that G will be determined by the available speed. So I think it is safe to assume that the initial decentralised will be slower than the conventional experience (inevitably as additional layers of abstraction are added.) The challenge will be to balance and optimise all parameters, and “usablility (limited by speed)” will be one of them.
As a bright spot, it is proven to not be impossible to get very usable speeds, by simply observing the success of the bittorrent network.