For a bit of context on the scale, the full plan for this constellation puts the number of satellites at about 4,000. The number of satellites (including the broken ones) currently orbiting the earth is about 4,000.
So of course initially it sounds insane, but satellites are getting much smaller and launches getting much cheaper. 2017 saw 50% more objects launched than any previous year. With growth like that, those numbers really aren't that insane. And even if the costs end up being 10, 20 or 30Bn, if that gives you a global, gigabit-class network, surely that's cheap?
What might be insane is the claim of 25ms latency and cable-equivalent upstream bandwidth. I'll believe that one when I see it.