But you could also compare it to investing in an early-stage startup. If the company you invest in gains popularity the value will increase. If you invest in the company there's an incentive to get other investors involved, as the more people wanting in on the action, the higher the valuation will go. And at time of investment, said company isn't actually generating any revenue, it's all just about future expectations.

Hence I would argue that it can be argued that Bitcoin is a bubble (i.e. equivilent to investing in a startup that is sure to fail) but not a pyramid scheme.