Lots of things take a long time and never become useful. One thing (the internet) took a longish time to become really useful.
One major difference here is that the grand vision for the internet stood to help governments in many ways, whereas cryptocurrency stands to weaken governments in many ways. Currency controls enables nation to try to tune their own local economies, control international trade rates, go infinitely far into debt, and can also work as a great weapon against enemy nations. Widescale cryptocurrency adoption would stand to shatter all of this, and that's something no major government is going to just idly allow to happen.
I would also add that in the zeitgeist of the world today, the internet would likely have taken far longer to materialize than it did in the past, for reasons analogous to those with crypto. Can you imagine the US's response to digitally interlinking with China or Russia? For that matter China or Russia's view on the same. We live in a brave new world where each nation wants to strictly control their messaging while limiting, or eliminating, outside views. The internet works to undermine all of that.
You need third parties to cash in and cash out, so no.
> Can you imagine the US's response to digitally interlinking with China or Russia?
Before the internet we had the phone network, and before that the mail. Both of those are more interlinked and more open than the internet is today.