Is it really the best way to invest your money?
Because of the way Bitcoin generates value or becomes more expensive, it could potentially lose all its value and became worthless.
Who assures you that this will not happen?
Nobody said bitcoin is the only and the best way to invest your money. You got that part wrong. You may decide to invest 1-2% of your total wealth and still be a crypto investor.
Your concerns are well grounded however. Bitcoin may indeed evaporate tomorrow without leaving a trace. It is written (or used to be, haven't checked the domain for a while) on the bitcoin.org website. It is experimental tech which nobody knows how it is going to end up.
Just because stocks went up higher during the last decade you don't invest 100% into stocks. You diversify.