> you'd be utterly insane, as an individual developer without a full-time security team, to expose a self-hosted application to the Internet.
You don't have to. The article mentions Tailscale--the whole point of which is to not have any Internet-facing app exposed. Everything is done peer to peer between clients that are behind firewalls. There's nothing listening on an Internet exposed socket for random connections to come in.
Read on in my comment for the more important point about fragmentation. Average non-technical users won't - and shouldn't have to - accept having to switch between different Tailnets to access different instances of similar apps hosted by different people. Heck, most average users would bounce off of the idea of having to install and use Tailscale in the first place.
(I use Tailscale myself for accessing my own private applications while on the go - but I don't believe it's a practical solution for generic widespread access)
You don't have to. The article mentions Tailscale--the whole point of which is to not have any Internet-facing app exposed. Everything is done peer to peer between clients that are behind firewalls. There's nothing listening on an Internet exposed socket for random connections to come in.