Apparently, this is not just putting Plex's own proxy/DNS service behind a paywall -- they will also gate some small features (such as the "custom server access

Apparently, this is not just putting Plex's own proxy/DNS service behind a paywall -- they will also gate some small features (such as the "custom server access urls") that are necessary for one to run their own reverse proxy and DDNS. As in, Plex will prevent you remotely accessing your own server, even if you're not using Plex's services for that remote access. So this is purely extractive move, and not really based on their costs at all.