Bitcoin,fiat and gold have their own purpose why they exist. Bitcoin is definitely the best in terms of investment because of the higher earning potential. And it is good to invest both for short and long term if there is really knowledge about crypto or digital currency. Gold is for long-term investment that has been around since then. Fiat is what we use every day, with or without bitcoin, the truth is that we need fiat for our daily life even if we want to invest both in the stock or crypto market. Those three act as a medium of exchange which means they are all important.
As you said everything has its own purpose, and also choosing Bitcoin doesn't mean we don't need fiat and Gold.
Because after all Bitcoin also has weaknesses and is not something perfect, so the presence of Bitcoin is not to replace fiat and Gold.
But Bitcoin was created as an alternative either as a payment or as an investment,
having all three is much better than having just one. Moreover, many countries still prohibit Bitcoin as a means of payment,
so fiat is still very much needed in our lives. Likewise with Gold, although Bitcoin can provide greater profits,
but it will be much safer if we diversify our investments.
So investing in Gold and Bitcoin is much safer than we only focus on investing in Bitcoin.