I can assure you that L3 and Cogent are not seeing the marketplace shrink. 10k quarterly files from each discuss their growths and vulnerabilities.

Historically, eyeball networks have paid for transit from backbone providers. As market consolidation occurred, the cable providers with government granted physical monopolies were able to negotiate for better settlements with their transit providers.

Eventually, the cable companies condensed with the major telephone companies. Some of the new mega companies have backbones and can use their own networks for transit (Verizon, AT&T, ...) others (Comcast) were able to use their size and access to their customers to negotiate largely settlement free exchanges.

However it is important to note that ALL eyeball networks have 1:5 to 1:20 demand ratios. This is the nature of content versus consumption. There is no new news regarding these ratios, and they are not particularly germane unless attempting to engineer the flows.

The real and pertinent issue is that Comcast has not lived up to the 'timely upgrade' clauses in boilerplate (NDA'd) settlement-free peering agreements. Clearly they have an advantage to "defect" from the standard cooperation model. Netflix has chosen to change providers several times, and recently provided data on whom they pay for service.

Why should an end-user ISP ever expect a settlement-free peering bandwidth graph to be symmetric? The important question is whether customers are requesting connections that route through through a particular transit provider. Comcast gets to sell "Internet" access, while the transit provider gets to sell bandwidth. Win win, no symmetry needed.

...you can't quantify "benefit"....

Isn't that the entire purpose of an economy?

You're right to call me out here. Although I have a reasonable idea what it would take technically, I don't actually know the financial impact to Comcast for increasing capacity to L3 or Cogent (not even ballpark). However, I'm taking the L3 article at face value when it says:

    But there are also typically shared costs for networks to 
    interconnect. Each party pays to augment its own network 
    to allow for more traffic exchange (the expense to augment 
    capacity is not significant for either party). And since 
    we often choose to interconnect in a third party data 
    center, the networks usually agree to share the cost of 
    the cross connects by paying for them on an alternating 
    basis.

I take that to mean the CapEx is insignificant and the recurring expenses are shared equitably. What's notable about that is that Comcast isn't even sitting down at the table to negotiate equitable terms for upgraded capacity. They're out and out refusing to upgrade without direct payment.

Upgrading that interconnect mentioned in the original article seems like just plugging in some $10k-$20k standard hardware and connecting a single extra wire. That kind cost doesn't matter really, it seems that they're hurting their interconnects intentionally for political reasons.