Think of it like a super rare pokemon card of Charizard. That’s valued at 1 Billion. Later on people care less about pokemon cards though so the value decreases. What they lost was the value of an asset.

This can be bad for the pokemon merchandise market though as people are less interested they buy less cards and other goods.

Later the popularity could go up again and the value of Charizard could go back up.

If they ever sell the card then thats when the price is actualized.

But in practice no company can pay a trillion dollars to buy a large company like amazon. But those companies make tons of profit over their lifetime which is why they are valued so high.