In my eyes the 2017 rally, which actually started in 2015, was very sustainable, it lasted for more than 2 years and brought the price from $200 to nearly $20,000, and even after the crash, the price was still tremendously higher than where it was at the begging. If this rally would be as "sustainable" as the 2015-2017 rally, we could look at reliable $60-80k price ranges in a few years.

It's only from the perspective of people who entered close to the ATH that rally wasn't sustainable.