The way to deal with this is to constantly do live events, and actually build organizational muscle. Not these massive one off events in an area the tech team has no experience in.
They've been doing live events since 2023. But it's hard to be prepared for something that's never been done by anyone before — a superbowl scale event, entirely viewed over the internet. The superbowl gets to offload to cable and over the air. Interestingly, I didn't have any problems with my stream. So it sounds like the bandwidth problems might be localized, perhaps by data center or ISP.
My suspicion is the same as yours, that this may have been caused by local ISPs being overwhelmed, but it could be a million other things too. I had network issues. I live in a heavily populated suburban area. I have family who live 1000+ miles away in a slightly less populated suburban area, they had no issues at all.