Listened to Saylor on Saif's podcast and he talked about this. It sounds like his assumptions are not based on the dollar collapsing or anything drastic really. Econ keeps moving, dollar is still the dominate money in the world. Basically bitcoin as an asset sucking up gold's market share and some of many other things. That's a pretty conservative view IMO. I suspect we will see a massive event in the next 20 years which could make bitcoin in relation to the dollar skyrocket. That said, I like Saylor's look at it because if you are using the dollar as you unit of account and you assume it is just going to keep inflating these numbers aren't crazy.

Now if we have a black swan bitcoin in relation to dollars could be even more massive but its purchasing power might be closer to Saylor's numbers. Its all just speculation though.