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
I think if we're talking about investment as such, it must be something that is developing and has a potential. It is such a situation with cryptocurrencies, USD isn't developing now, as long as it's been existing since 1861. By buying USD you can just support its existence, but what's the profit? It will only help governments to control people's money.
I personally not going to choose USD or any other fiat currency for investment purposes, because i know that they have very stable price and a very small fluctuation can be observe in their price, instead i will like to invest my money in altcoins where we can choose such altcoin who have more potential and their price are expecting to increase too much.