As for your concerns, bitcoin will crash since long time ago if there is no substantive thing, but as you can see now its price just make many people regretted to not buying it at the low price.
More than that, if bitcoin doesn't have a function then I guess there is no significant increase from its lower price $3500 because previously its price had been increased to $20.000 and crashed to $3500 but as we can see now $56.000 is a high price for an investment place.
There is always a function for bitcoin, even if we can say "running away from fiat to keep the value of my wealth" that would be enough, it is called store of value, if you can buy a house for a million dollars today, and instead you keep that million dollars in bank, in 10 years that house will be maybe 3 million dollars and your million dollars will not be able to buy that house, that is inflation, and with a store of value asset you put it in there, and it will at least give you whatever that house will be worth, maybe more as well like bitcoin, there could be lower periods as well when it goes down but if you wait enough it will break through that and go higher.
Think about buying at 20k and thinking how stupid you are for buying at 20k when it reaches 3k, but if you kept buying instead of being upset, it is 56k+ right now, you would have been richer. Hence the store of value is a valid reason, even if no other reason is given.