I offer a cultural definition of "Mainstream Adoption"...

When you can talk about cryptocurrencies with just about anyone and it is considered "old hat" and not new anymore, it has been adopted by the mainstream.

The Internet is good example to compare to. While almost totally unknown 25 years ago, it has been incorporated into the fabric of civilization. There are plenty of people who don't own computers and go on the Internet, perhaps not so much with young people, but suffice it to say that there are billions of people out there who have no direct involvement with the Internet, but their country is served by global Internet communications. The Internet is still mainstream in this case.

When it comes to mainstream adoption and crypto, I think the blockchain being ubiquitous in the corporate world will signal mainstream adoption. The blockchain IMO is not going to be a direct boon to the individual like the Internet was (Steemit is an exception). Google makes anyone an Oracle with magical powers. Anyone can get instant access to information that took weeks, months or more to research only decades ago.

Distributed ledgers require 100s of computers and the Internet to make them work. This is infrastructure suitable for corporate level management, not a tool for the individual.

I could be wrong (and I hope so) because in the future we may be seeing many as yet unimagined things come about in this milieu of exponentially accelerating technology.

Your article brought up a question that has been on the minds of many, so I have upvoted and re-steemed it