The warning not to put all your money into BTC is also still correct. Just like it applies to any other investment: diversification reduces the risk of huge losses.
Bitcoin is not an investment, it's a currency.
None is going to call USD or EURO is an investment.
So you want to call it "savings" if you keep Bitcoin? Fiat money isn't an investment because it continuously loses value. Bitcoin, although designed to be
peer-to-peer electronic cash, is mostly used as an investment nowadays.
I wouldn't recommend to keep all your money in fiat either.
Bitcoin is volatile, the price moves against what? 1 sat = 1 sat.
I used to say "1 BTC = 1 BTC", until I realized that's like saying "1 USD = 1 USD", and it's not true. That's why USD-prices are usually related to a year: a dollar in 1960 was worth a lot more than a dollar now. A Bitcoin in 2010 was worth a lot less than a Bitcoin now.