Yes and no, depending on your plant build out you might have great interplant connectivity - 10G between each switch, 10G to the router, but the extraplant connections still cost and have bandwidth limits. So if you only use 2 gbps total during peak, you may not need to upgrade your plant to support higher speeds but you will have to pay more to your upstream provider to support it.

This is actually relevant to my ISP day job, we have a pipe to a well known backbone provider, and dual pipes to a statewide provider. We ran on the dual pipes for a long time before pushing our ISP traffic over the single backbone provider. These days, if that main ISP pipe goes down, it fails over to the dual pipes to keep everything running. But now that we are offering packages higher than 25/3 for DSL, terrestrial wireless, and cable, the dual pipes don't have enough bandwidth to keep up.

If we offer gigabit service over fiber (and we do) just a couple of customers actively using their whole pipe is an enormous chunk of the network compared to the 40 or so customers that same gigabit of bandwidth would serve on previous 25/3 packages. We don't have data caps or even contracts (small town benefits :P ) but there is a lot more to it for smaller ISPs than adjusting the rate limiting and packages we allow people to use.