Even the Winklevii's famous lofty predictions for BTCUSD are absolutely logical & plausible SHOULD critical mass adoption be achieved, somehow, as you mention. I don't have the slightest doubt.
..The problem lies in reaching that kind of exponential userbase growth in the first place.
You can't.
You'd need to fix the following three factors :
A] [Demand] Create a zeitgeist movement convincing/marketing to the mainstream public that banks are not safe, not your friend, they control your funds and you have little recourse, whereas trustless crypto like Bitcoin allows you to have full control, be your own bank.
B] [Security] Make wallets & payment gateways/crypto website GUI's a lot safer to use, idiot-proof, insured, and user-friendly/less technical-inclined.
C] [Access] Last but not least, exploit most people's laziness by paying them in BTC instead of fiat. Now that you have tens, possibly hundreds of millions of employees being paid in Bitcoin for their regular job's income, as long as there is even more merchant adoption for them to freely, easily spend it at - Will they bother switching their 'crypto-paycheck' every 2 weeks, back into fiat..? Of course not. They don't buy BTC right now because their main sources of income come in fiat. The minute you switch this around - they receive payment in BTC instead, on a regular basis - then they don't bother swapping it to fiat.
And even then, you'd be wrestling against :1] The mainstream proletariat is lazy, dumb, complacent ; lives in a complete oblivious adherence to a sheep mentality comfort zone and culture of lowest common denominator mediocrity.
2] Banks, Wall St, and Big Tech corporations want fingers in the entire crypto & blockchain pie as widely hedged & spread out as possible, not just BTC - And they've already done a big share of the investment interest they were going to do, by now.
But most importantly, when they want to blockchain up, they create their own. They have the resources, the power, the money. They have the bias, the policy, the legal incentives even, to create their OWN centralized permissioned blockchains & brainwash the public in using / trusting them instead (and that's exactly what they've been doing so far), because that's what people are used to and human nature/core pillars of industrial society's cultural norms don't change overnight like that, just because 'Oh BTC is pumping..'
3] Governments are self-perpetuating power control cartels that function through extortion.They don't go decentralized or venture in a non-mainstream tech vulnerable to Majority Hashrate Attacks, protocol malfunctions, and openly manipulated markets without a central bank they can control/blame.
They also don't care about money. They can print infinite amounts of it. It's a widely held misconception but nonetheless asinine that the extremely powerful major governments in modern times care about seizing funds ; When this happens, it's primarily a deterrent against other citizens/companies breaking the law, and allows their Treasury a tiny bit more breathing room vs. having to print more at some point - which risks unbalancing economics towards an unwanted amount of inflation, and even public discovery, which would harm the one thing governments DO care about & make their primary 'currency' to deal in ->
APPROVAL RATINGS from the MAINSTREAM PUBLIC as that is what gives them such
power -
NOT any form of
money.
So this means no, they won't pump your BTC or any decentralized 'un-ruled' cryptocurrency ; And they won't declare it legal tender.
