If the technology cannot provide unlimited for customers who have paid for it, then unlimited service should not be sold. Simple as that. Anything else is inexcusable. If the technology stack can only handle all customers at 250 kBps, then that is the maximum service they should be selling.
"We can handle 20% of users at 5 Mbps, and the other 80% at 250 kBps. So we'll sell the 5 Mbps. But we won't restrict sales to 20% of customers, we'll sell to as many as want it and then throttle to handle overloads on the system." There is no way to make this situation acceptable. Don't sell what you can't provide. Pretty fucking simple.
"We can handle 20% of users at 5 Mbps, and the other 80% at 250 kBps. So we'll sell the 5 Mbps. But we won't restrict sales to 20% of customers, we'll sell to as many as want it and then throttle to handle overloads on the system." There is no way to make this situation acceptable. Don't sell what you can't provide. Pretty fucking simple.