What you're asking for already exists. They are dedicated business circuits. I get where you're coming from, but there is a reason that the consumer advertisements always say "up to X mbps". If ISPs didn't oversell, consumer Internet would either be unbearably slow or obscenely expensive.
A guaranteed 1 gig circuit is on the order of thousands USD month. An oversubscribed consumer fiber provider like Google Fiber is an order of magnitude cheaper because they can oversubscribe based on consumer traffic models that cover 99% of users.
Also keep in mind that your model isn't really straightforward at all. The ISP can only provide you guarantees within the lines they control. There could be any number of choke points at IXPs that mean the user can't reach site X at their guaranteed rate. So ISPs never even offer hard guarantees on connections to the wider Internet. The most you will be able to buy is dedicated circuits to their core, an IXP, some branch office, etc.
Are you okay with gyms selling more unlimited memberships than the number of machines they have?