There will be no guarantee. But there will be multiple service providers for people to choose from. Competition between service providers will push consumers to the better service.

For example, suppose Verizon obeyed net neutrality whereas AT&T did not, favoring instead their own content. Consumers would naturally choose Verizon because their connections were not being throttled. If enough consumers move, it will force AT&T to respect net neutrality as well.

In Canada our regulator recently discussed this and one of the largest monopoly ISPs went to bat in favor of net neutrality. I suspect this is in large part due to the great increase in competition from the leased lines being enforced.

If all the other companies aren't doing fast lanes and are doling out more bandwidth, well, you better compete, because that's something people will notice pretty quick.