I think that's a good lower bound, as it assumes no packet scheduling, and that everyone on Earth will be attempting to consume all the bandwidth available to them all of the time.
It also assumes that the "backhaul" between satellites wouldn't saturate under such a load.
I'd wager that under those assumed numbers you'd more realistically be able to expect somewhere between 100Kbps and 1Mbps.
the first problem is the evening peak when everyone tries to get on at the same time and watch netflix/youtube. The second problem is the link equation from your hemi antenna handset to the satellite 1000km away that passes by in 100s.
Yes with packet scheduling you can get higher peak bandwidth, but 1Mbps sustained over the course of a 5 minute youtube video is going to be hard.
It's much better to target the whatsapp crowd. That facebook will pay for.
It also assumes that the "backhaul" between satellites wouldn't saturate under such a load.
I'd wager that under those assumed numbers you'd more realistically be able to expect somewhere between 100Kbps and 1Mbps.