Yeah, no.

Working around NAT was trivial for the people who actually cared about it. I was adding port forwarding rules to my parents' router at age 12. Turns out exposing a poorly-configured Windows XP box to the wider interwebs is a Really Bad Idea - and for the same reason UPnP letting random unpatched shady P2P applications do the same is Very Much Not Good.

Let's face it: consumer devices simply aren't secured well enough to let the entire internet poke around in them, and it was even worse a decade or two ago. Decentralization is pointless when it only results in people compromising their own machines, and the people with the skills to set up a 24/7 Linux server in a broom closet won't care about adding some NAT forwarding rules.

Even without NAT, we would've definitely gotten home internet routers firewalled with a default-deny policy on all incoming connections. Exactly the same "manually configure a bypass, or use UPnP" dance blocking you from trivially running a web-available service on your machine, but with a firewall rule rather than a NAT port forward.

It's of course a different story with CGNAT, but that only became a thing well after the internet was already centralized.