If there merely exists anything which is more valuable tomorrow than today, it is also equally "stupid" to have cash instead of whatever that thing is... and like, that isn't even theoretical: you are generally encouraged to hold more random assets than cash (if you are allowed to and if you can handle the volatility, of course).

The reality is that, even if the stocks you own are going to the moon, if you need food or want a television, it isn't at all "stupid" to sell some of your hoard to buy stuff.