I really appreciated OP views and others that work hard to think about pitfalls. He has some good points. As for my thoughts:
1. IT'S A DECENTRALIZED CURRENCY
Not a valid reason for failure. Gold is decentralized--controlled yes, manipulated yes, but largely cannot regulate effectively, even though USA has tried in the past. Bitcoin won't fail because it is decentralized. If anything, it guarantees success just maybe not at the level you are imagining.
I suppose if the USA issued a statement that you can go to jail if you issue a BTC transaction of any sort, then it would be doomed, but then I think you have rebellion and we are doomed anyway.
2. CENTRALIZED MAIN DEVELOPMENT
Not a reason for failure, but a risk. I have always argued that you can be an atheist and hate religion and the manipulationg and control it represents, but then I say you are probably putting your faith in a different group of priests, which can just as easily create their own tyranny.
I think the same idea applies here. You can hate the government and believe bitcoin is the answer, but ultimately we are putting our faith in a different group of politicians. However, if anything this is a harbinger of success not failure, global domination must be organized, planned and executed, so bitcoin foundation get to it.
3. FLAWED ARCHITECTURE
Yes could cause failure if 100TB is necessary tomorrow, no will not because it is software and we have years... if they can fix healthcare.gov bitcoin blockchain will evolve and remain decentralized. Guaranteed and I know nothing about the ideas anyone has ever thrown around on this.
4. Double Spends
Same as as #3 and a stretch anyway.
5. Slow Speed
Bitcoin does not need to replace fiat to be worth $10k a BTC, it only needs to be considered relatively liquid, similar but rather different issue.
6. Un-Scalable
Same as #3, and #4, if
51% Attacks
A risk, however, why would they raid their own hen house. It this happens, it would be because of sabotage from unseen forces like Ed Snowden's boss.
Unstable Exchange Rate
Definitely not a failure point. It will solve itself over time. Gold is unstable but will always be valuable. I lost 5k in gold value this year alone.
4. NO BENEFIT OVER FIAT
BTC never needs to replace fiat, and it does have benefit your post eludes to a few. BTC is its own class, I agree that right now a regular currency it is not.
5. CONCLUSION
No real red flags for failure to me that I see.
I don't even care about how many Bitcoin users there are. How many individual investors are there in AAPL??? Likely a few million. AAPL is worth 500+ billion i think. That is one single puny stock.
When I mean fail, I meant fail as in mainstream adoption it can continue if it stays at these levels without problems, but everybody seems to want to talk about how Bitcoin is going to take over the world which it will fail to do so for the reasons above.
Yes gold is decentralized and it clearly proves point #1 hence why it is not a currency, just as a store of value. Again the argument is for something to be adopted mainstream. Anything decentralized can succeed on a small scale, gold and silver for example, people can decide to transact with it.
But on a massive scale, when 90% of the means of production is owned by 5% of the worlds population, they will not let it become mainstream.
As you cannot go to the bank and transact in gold and silver, nor bitcoin, nor decentralized post stamps, carrots, bottle caps. Even if you could multi billion dollar companies which we all rely on for our daily day to day activities will not accept our new methods of payments. If we cannot pay our basic bills than the "decentralized good" is basically good as dead for mainstream adoption.
They own everything and they have zero plans of giving their control up. People can try and fight as much as they want, but when they can control your basic necessities of food, clothing, shelter your not going to get very far.
Only way for true change to occur is for a massive reorganization at the government level, than if the government is taken back, they can go after these corporations and these people, and levy criminal and civil charges and take control of everything back. Then we can open up the doors to decentralized currencies.
Good Luck with replacing the whole worlds governments, people barley vote, yet run for office and be deeply involved with politicians and candidates to enact this kind of change.