Your network authentication should not be a fun game or series of Rube Goldberg contraptions.
Also by collecting data on the IP addresses that are triggering fail2ban I can identify networks and/or ASes that disproportionally host malicious traffic and block them at a global level.
Logging both successful and failed requests is important for troubleshooting my systems, especially the client-facing ones (a subset of which are the only ones that are accessible to the open internet), and failed authentication attempts are just one sort of request failure. Sometimes those failures are legitimate client systems where someone misconfigured something, and the logs allow me to troubleshoot that after the fact. That it can also be fed to fail2ban to block attackers is just another benefit.
> You can't meaningfully characterize attacker traffic this way. They'll come from any AS they want to.
Obviously in a world full of botted computers, IoT devices, etc. it's true that an attacker can hypothetically come from anywhere, but in practice at least from the perspective of a small service provider I just don't see that happen. I'm aware that you are involved with much larger scale operations than I'm likely to ever touch so perhaps that's where our experiences differ. No one's targeting my services specifically, they're just scanning the internet for whatever's out there and occasionally happen to stumble upon one of my systems that needs to be accessible to wherever my clients happen to bring their devices.
Sure, I see random domestic residential ISP addresses get banned from individual servers from time to time, but I never see the organized attacks I see which are usually coming from small hosting providers half way around the world from my clients. I have on multiple occasions seen fail2ban fire off rapidly sequential IP addresses like xxx.xxx.xxx.1 followed by xxx.xxx.xxx.2 then xxx.xxx.xxx.3, or in other cases a series of semi-random addresses all in the same subnet, which then triggers my network block and magically they're stopped instead of just moving on to another network. If I were to be packet sniffing on the outside of the relevant firewall I'm sure I'd see another address in the blocked network trying to do its thing but I've never looked.