Having a bandwidth cap is basically the ISP punishing you for contending too often.
That said, assuming the ISP wants to keep an average speed higher than the minimum their contention ratio would offer, people who download a lot would have a greater impact on that, and could therefore cause the ISP to have to invest in more hardware and back-haul.
The counter is that the ISP isn't upgrading their network anyway, they're just juicing the market by charging some people more.
Most ISPs are constantly upgrading parts of their network. Earlier this spring AT&T came through my complex and installed fiber in every apartment. Average broadband speeds have increased significantly over the last decade, and for that to have happened significant network upgrades would have to have taken place.
To my mind the question would be "if an ISP has network with a fixed capacity, do their monthly operating costs differ between a month where they consistently utilize 50% of that capacity compared to another month where they utilize 80%?"
I suspect the cost to the ISP is the same in either case: they have purchased the equipment and bandwidth to support the network and those costs are fixed, regardless of how much of that bandwidth they are actually using. If that is true, this idea that there is only so much data per month to go around and if you use more then your neighbor then you should be charged more money would be fundamentally dishonest.
(1) This is overly simplistic; these are individually-negotiated B2B contracts, so details vary widely.
Based on this article, I suspect that ISPs like Comcast and Charter are not extending their backbone to handle growing internet traffic. It seems they are charging exorbitant fees to discourage increased usage instead (in this article, the subscriber canceled their Hulu account and lowered the resolution of video supplied by Netflix).
See also: https://en.m.wikipedia.org/wiki/Tier_2_network , https://en.m.wikipedia.org/wiki/Tier_1_network
If there is truly demand for unmetered internet someone will step in and fill the void. Some ISPs also use total transfer as a way to discriminate between business internet and residential internet.
It's a giant bucket of 'it depends'. If the CDN network serving the content has placed hardware within the ISPs network, it's likely a non-issue. This was a major issue before folks like Netflix started moving their CDNs closer to the customer. You might have a hard time finding the articles, but I recommend looking at the pissing match Level 3 and Verizon got into years back.
>it would happen if caps existed or not
I'm not convinced. You are essentially arguing that scarcity of a resource has no affect on consumption.