as late as 2015, 2.1 million people were somehow still using its dial-up service
That number is how many AOL Dial Up customers there were in 2015. It's from this article - https://www.cnet.com/news/more-than-2-million-people-still-p...
Paying for the service is not the same as using the service. There are many reasons to pay for a redundant dial-up connection in case of an outage with your main internet provider. There are probably lots of people who had a dial-up account but forgot to close it. Some people probably did still use it, but for things they don't need broadband for (email, IRC, etc).
You can't use the existence of a handful of dial-up connections to claim we should be building web software for people on dial-up. That just shows you haven't thought about things enough.