Depends on your perspective and what you believe in.
If you believe bitcoin will replace gold as the world's foremost storage of wealth, you probably believe bitcoin will be worth $400k+ in the future. If you believed that, I'd argue you'd be a fool not to buy it at $40k with a 10x return potential.
If you believe bitcoin cap is $40k, I wouldn't buy right now.
Depends what you think
As of this writing bitcoin already hit $40K I really regret not buying last year when it is still around $27 because by this time there is a big chance that what I have invested last year could have been doubled. We cannot really call them idiots because of how far bitcoin price can achieve.