True, but it's more a case of a small number of users resulting in tremendous loss of profits.
Suppose you open a restaurant and offer free bread and some customers come in and buy the cheapest item on the menu and eat $10 worth of bread. It's just not sustainable.
Which is why no restaurant claims to offer unlimited free bread.
There is no "but". If it were actually true that a small number of users consume so much bandwidth that it causes "tremendous loss of profits", then ISPs should drop the "unlimited bandwidth" claim.
But they want to keep that because it's just so nice for marketing.
What they really want is to claim "unlimited bandwidth" without actually offering it.
Suppose you open a restaurant and offer free bread and some customers come in and buy the cheapest item on the menu and eat $10 worth of bread. It's just not sustainable.