There's a vast gulf between running a server on the public Internet and writing a modem script. When writing modem scripts was necessary to get online very few people actually bothered getting online even if they had a computer capable of doing so. Running a server of any type of the Internet is rarely a trivial exercise. Even when running one on a home system from a residential ISP you've got to poke holes in your router's firewall and maintain the server hardware and software. These are not things most people are even capable of doing let alone being interested in it.
> So perhaps the centralized platforms can't be avoided, or perhaps we need to make running your own server easier? And maybe both ways can co-exist, because different people want different things?
No matter how turn-key you make "running a server" there's going to be technical issues many people won't understand or be able to handle. We already live in a world where turn-key Internet connected devices are exploited en masse by malware.
You're also ignoring one of Moxie's major points that a huge portion of the population gets online via mobile devices. Just the basic design of mobile OSes largely precludes long-running server software running in the background on them. They're also on CGNAT networks with varying IPs and roaming on and off various WiFi networks. The only meaningful way for them to serve content is to bounce it off of centralized servers that don't have the same limitations of power or network connectivity.