those are extreme cases though
no one in their right mind will spend something that might be worth double the next day if they can wait a day with their purchase.
deflation is never that extreme though.
in fact, currently we still have inflation, and technically will keep having inflation until 2100ish. (although it will likely stabilize sooner)
since bitcoin will not be destroyed that often (aside from a few accidents) the value of bitcoin will rise and fall with the global economy if it becomes the main world currency.
so bitcoin will be worth more if the economy is healthy, and worth less when the economy is unhealthy.