if the countries vote laws making illegal to use BTC as exchange in any shop.
TOday the price goes high because many countries enable it as an exchange currency (it is not only used on darknet for escrow)
But if tomorrow countries decide to control it , they will FIRST forbid to buy things with it.

....and if you cannot exchange it legally ,...it turns back to the darknet time ...when the price was a lot lower.

What makes you think it will value that much ? Just the past evolution ? The fact that it is limited number?