It really is this simple: put a $100 bill on the table, and leave it for 100 years. How much purchasing power is left? And compare that to gold or Bitcoin. This is the store of value of a money, period
You won't live for 100 years, end of story. Well, in fact you may, but then in the end of this timeframe, the purchasing power of the 100 dollar bill will be your least concern as I come to think. And technically, you would be proved wrong if you were to actually put that bill on the table 100 years ago as it would acquire quite a lot of numismatic value by now, so in the end, your purchasing power would have actually increased multifold (provided you would still care, of course)
And it is not given that Bitcoin will still be around in just 10 years from now on
The bottom line is, fiat money is a terrible store of value over the long term. End of story (as you say.)
When you have to claim store-of-value by using numismatics, it's time to say the system is broke.
Fiat money is not a long-term store of value. If it were, it would not be good as a currency. In this manner, your whole point becomes kinda truism (read, you should spend it, not save)
This entire argument assumes that the powers who run the system have our best interest at heart and have been correct to force people to spend or take risks with their savings. This is the true face of central planning. The final results of central planning are theft, deception, and murder.
Money, and fiat money in particular, is a tool and how it is used and to what end depends on who is using it and what their goals are. In other words, don't shoot the messenger
'Fiat' money, by definition, is controlled by the political and financial elites (issued 'by fiat.') As soon as that power is concentrated, the incentives for the elites make sure that theft, deception and murder will happen.
The fruits of this unnatural growth is time poverty leading to all kinds of unhappiness, addiction and mental illness. And this is among the lucky. The unluckiest of the world get their family members and homes blown up by bombs because the system incentivizes the elites of the dominant imperial countries to use wars and 'regime change' to force other countries to support the system, directly or indirectly
Well, you can't actually expect people to be any different. As they say, charity begins at home (just in case, Bitcoin is no exception to this rule of thumb)
True, the only way for real change to happen is that we realize 'you can't expect people to be any different,' and take the power of money away from the elites. (What role Bitcoin plays in this is actually not very important.)