Miners control the majority of Bitcoins and the large majority of people involved in the Bitcoin community are strictly Miners looking to exchange Bitcoins into $
Barely anyone in the community listens to themselves.
If the majority of BTC holders/owners are looking to exit BTC for real currency, then obviously there will be a finite number of "trading places" cycles of BTC seller looking to exit the BTC holding status vs new market participating looking to enter the BTC holding status.
Unless the BTC "economy" were to offer a service(better: more than one) that DOES NOT EXIST PURCHASEABLE IN ANY OTHER SHAPE OR FORM OUTSIDE OF BTC, there is no reason for someone to have or hold BTC if they can come to attain the service either more safely, quickly, commonplace, securely, etc pp by means of normal currency(exception: other significant advantages in some shape or form that also get recognized by enough "outsiders" to gain traction and make the adoption of something new instead of something established more viable/attractive).
The exchange to BTC itself already carries significant risk (like buying tokens for an arcade - the funny money only works in wonderland, and if you leave it before spending it all, it's worthless weight in your pocket), and is only mitigateable if the risk of a sudden disappearance of BTC markets or value is low or nonexistant. Currently still, you can at least cash out again, even if there are value swings.
All of these basics lead to the shaky foundation that BTC currently stands on. Until you find a whole bunch of creative/innovative/unique people that will only offer their goods and services for BTC, you really have no compelling argument why BTC should work / be used.
And if really what you are inferring were/is true, and the only main/major purpose is speculation and then cashing out, then at some point the jig is up and the constant outstream of BTC for capital will so heavily outweigh the market structures, that prices will go bust and the buyers will vanish to negligible to nonexistant amounts, as they have nowhere left to go / no more reason to join the game.
This is also why people calll it one big ponzi scheme; if that assumption is correct, then you need to constantly exploit either the BTC using morons that keep buying more BTC in hopes of them ever growing more valuable(mostly through the moron next to them ALSO buying them for the same reason..look guys - who is going to cash both of you out if you are the only two guys left with all BTC in the end? Now just extend this thought to the speculator pool in total..), i.e. you keep praying the guys who have thought it a great investment keep working in the real world for real money to donate to those cashing out of BTC, or, option B, you need an influx of people suckered into the concept via media hype.
However in neither version is there an endgame that does not end with those that have exited before the music stopped being the lucky ones, and a select few with literally most if not all of the BTC left in their hands without much of a counter-value or cashout option.
I don't really see how this is solveable, and I've already heard all the "Yea, but there's people already accpeting BTC for xyz.." and "If only 10% of the world adopted BTC" delusions..
From the OPs own post he basically lays out a premise that leaves no other option than deck of cards => mess.
Of course if someone DOES find some kind of magic bullet killer app/use/whatever that all of this may or not be good for, maybe you can build something sustainable. Others have found ways, too(second life, MMORPGs, other pseudo-"ecosystems" in form of entertainment and services), so it's not completely impossible.
It's just a big empty slate and giant question mark as of now.
I still maintain that unless someone comes up with something along the way, there is a very real and constantly growing risk of the cycle running out of "feeding cash".