I like that model for me, but here's why the ISPs will never adopt it: they would get less revenue from the vast majority of their subscribers.
They already know these people will shell out $50/month for their connection, which they do not use to its full potential. For example, my parents get cable internet and use it for basic stuff, checking email, stocks, basic research on things, maybe the occasional YouTube video that someone forwards to them. They couldn't use more than 10gb a month. Under your plan, they'd spend something like $10/mo. The cable company is never going to go for that when they know they can extract another $40/mo from them.
Now lets say you raise those rates so the basic users pay just about what they pay now. That's going to raise the monthly bills for those of us who use our connections substantially, and possibly price us out of their service. I'm still starting out my professional career, I can afford the monthly bill for my internet service (though I'm still at a promo rate) and I could afford to go somewhat higher than what the real rate is going to be after the promo expires, but there is a point where the price exceeds what I can justify for the value I get from the service. I could use it more like my parents, but that reduces the value to me. So essentially, with your plan, they lost $40/mo and could lose the $50/mo from me, a net loss for the provider.