So far, every fiat currency in the world is inflationary. It will always decrease in value overtime in the long run. Its a guaranteed loss !
Whenever you sell bitcoin, you are buying a fiat currency; making sure you will lose in the long run
TLDR: JUST HODL BTC
Im not getting the core point here. Why would you relate both the worlds in first place! The circulation for the bitcoin is fixed and whatever prices it is getting is getting because of the fiat currency only. If BTC = $1 and the market value for that currency drops then it doesn't really mean a lot to the BTC because after that also the BTC is $1 and whenever you will sell it, it will become the 1 USD only. Correct me if I'm wrong here.
Whenever you sell bitcoin, you are buying a fiat currency; making sure you will lose in the long run
Its not really investing into the fiat currency but its the output of the BTC sell in terms of specific value which it had at the time of sell. Later you can utilise the same currency in the real world and everything is bought according to their value. So its like defined values everywhere.