I thought about what you said and I made a different calculation.
We have a market cap of $60B and it is estimated that around 5M to 10M unique people are using Bitcoin today. So let's say 7.5M people.
$60B / 7.5M people = $8,000 per person.
$8T / $8,000 = 1B people
So to reach $8T, we would need to expand the Bitcoin community to about 1B unique users. That sounds like a lot, but is it impossible? I don't think so. Facebook was able to build a user base of 2B unique users in a time span of 13 years. (and reached the 1B mark in 12 years) WhatsApp has been able aquire 1B users in 7 years. So it is very much possible for disruptive technologies to experience such an explosion of users.
I hear you and agree that disruptive tech can adopt users at an explosive rate. However I would argue that those had very easy-to-see utility for just about anyone at any age. You don't have to know anything to open a facebook acct - just sign up and start using.
On the other hand, Bitcoin still remains very misunderstood and you have to worry about private keys, wallets, cold storage, exchanges, masternodes, different exchange rates, etc. In other words, Its not that easy yet to get into.
In addition, the crypto market also doesn't have "a face" ...something that brings the market great news everyday. All the average joe sees is the wild bitcoin swings.
I'm a firm believer that blockchain will be a large part of the economy one day. 2017 was an amazing year for crypto. Its not hard to see the crypto market hit 1 Trillion within five years. 8T in eight years is possible but a lot has to go right for the average joe to believe in blockchain for that rate. I think 10-15 years we'll see the numbers you want.
Lastly don't forget that your numbers are for 16T (8 of which is bitcoin). That means 2B people total. In addition you are saying that bitcoin will hold the same amount of weight 50%. I can see bitcoin ruling for a while...i'm not so sure in the long-term.
I agree that it is still hard for the average Joe to understand Bitcoin and to use Bitcoin. But we cannot imagine what kind of platform may adopt Bitcoin in the future and may mask all the difficulties of Bitcoin from the average user. Eventhough I do not like centralized platforms taking control of the aspects of Bitcoin which were intended to decentralize and give the users full control, it might help Bitcoin get mass adoption. Imagine platforms like Facebook or Google Play Store or Apple App Store adopt Bitcoin as payment option and make it the preferred payment option... this could lead to mass adoption. Or it could be a new platform which doesn't exist today. 8 years is like an eternity in the tech world. I think it is conceivable.
Ha - agree: things can move very fast in eight years in tech!