Your argument would apply equally to any currency. What stops me printing jstanley dollars and claiming there is 1:1 parity with actual dollars? Nothing! And yet that doesn't devalue actual dollars.
If most people had legitimate reason to believe that your jstanley dollars would retain 1:1 parity with US dollars, and they would be able to be used interchangeably with US dollars for most financial interactions for the foreseeable future, then you might devalue the US dollar by printing enough jstanley dollars.
That is unlikely though - there is no reason to believe any of those things about your dollars.
Whereas if you believe in the model of bitcoin, why don’t you believe in the model of bitcoin2? The best answer I am aware of is “network effects”, but that isn’t a very good one.
Bitcoin, or the technology of Proof-of-work and immutable history that is based on it, is not like gold:
One can take the entire history, change specific parameters, and create a new coin (bitcoin cash for instance) that has the same history. Hence people that had bitcoins have the same amount in this new currency.
This is like taking all gold in existence, magically duplicate its exact quantity into say another chemical element which never existed before, and distribute the amounts to all the people that owned gold before; in the same proportions.
Would this make the scarcity property of gold less interesting for using it as a medium of exchange? You bet so! One could magically do this ad-infinitum, possibly creating infinite other chemical elements - what would then set gold apart from the rest?