Comment by vkou
The problem is that there are two parties in any transaction, and power users/casual users swayed by marketing will be the ones that will choose the medium on which the transaction will take place.
So while you may want a self-sovereign exchange, your counterparty doesn't give a shit about your preferences, or is actively happy with using PayPal (Because their dispute resolution is better biased towards their side of the transaction, or because they just never gave it a second thought.)
One can argue about the difficulty of overcoming network effects to make mediums of exchange viable until the cows come home but that's orthogonal to the issue GP raised and ultimately impossible to predict.
As a counterpoint to the rhetorical impossibility of a challenger overcoming an incumbent (in almost all subjects this is true, despite challengers in the world regularly overcoming incumbents), exchange rates indicate markets are positive about the network effect threshold being overcome, since if it is impossible to overcome, the value of all cryptocurrencies combined is approximately 0.