So with our currency if it becomes wildly deflationary with a single penny buying a car--how do you buy a cup of coffee? That is the practical problem with deflation. With bitcoin it can always be made smaller.
But the scenario you're citing isn't the central issue when it comes to deflation. Deflation leads to a negative spiral because it pays off to defer any kind of spending - thereby slamming on the economic brakes. Notable example is Japan in 90s - deflation has been a major factor in its meagre growth then and even into the 2000s. The BoJ's unwillingness to adopt a sufficiently expansionary monetary policy likely exacerbated the problem