Cannot is incorrect. Wantnot it is.
Why would i change my Bitcoin into useless fiat to buy me useless things i don't need?
But take this with a grain of salt, because i am very different in spending behaviour.
The fiat you turned some of your bitcoins into will only be useless if you spend it on useless things, if you spend the fiat on important things, it will be a good choice. The excess of profit from investment in bitcoins after it goes up can be turned to fiat and used to get other good investments like an investment in Real Estate etc. Real Estate also appreciates in value with time, so it is good for investment. By using some profit from your bitcoins to invest in Real Estate, you are diversifying your investment, and that is not something you will not want, or something useless.