IMO, the growth rate for bitcoin would need to decline strongly (or retreat over a long period) to truly invalidate the exponential growth model.
Your subjective bias is fooling you here. Mathematically false because there was never a mathematical confirmation that the model was correct in the first place. We are simply picking straws blindfolded unless we have historically complete examples and or some other well supported logic from which to argue which model should be correct.
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In other words there is a category error here.
Yes, I see your point and you might be right. There's probably some more advanced ways to analyze the question of "what is the most accurate growth model?" objectively, as well as the question of "what would it take to invalidate the exponential growth model?" But I don't have the skillset.
For example, perhaps we could assume the adoption process is exponential Brownian motion [like the plot I showed earlier], estimate the hidden values for mean drift and volatility from the number-of-TXs-per-day time series, and calculate some sort of p-value for the fit. But I'm way out of my depth now. I wonder if there's anyone here who has a deep understanding of SDEs and things like the Black Scholes equation, etc.