Hi. You're absolutely right.
Getting high traffic customers like Imgur allowed us to negotiate better commits, establish new peering and interconnection agreements, and otherwise lower the "unit costs" of the network we built.
It was generally never worth it to do this for free, but you're spot on that in certain cases it was worth it to do it for less than (current) per unit costs.
though not as often today as in times past, there are points where 10Gbps is more expensive overall than 20 or 30Gbps.