Instead, imagine the DDoS landscape if we had to pay a small price for bandwidth. There would be a natural disincentive to having a toaster saturating your bandwidth as part of a botnet because it would quickly show up on your bill. And something as simple as shipping an IoT product or Rasberry Pi with bad default username/password might suffer bad reviews like "1/5 stars, this product immediately raised my internet bill."
I know it's not perfect and most of us have a bad taste in our mouth from paying out the wazoo when bandwidth is priced per Gb, but it can be a fair system if priced well that fights against our botnet reality where we basically have zero insight when our networked devices our compromised.
I can also imagine better tooling provided by our ISPs in this world where they help us track down and itemize our bandwidth costs. "Honey, why is SmartToaster89 costing us $24 in network fees?"
It's impressive how poorly our current system equips everyone except malicious actors. How many ISPs don't even filter spoofed outbound packets?
It's hard to complain about everyone centralizing around Cloudflare with the state of cheap DDoS muscle.