I agree a tool like that would be nice, but I’m not quite sure there’s a lot of benefit to it; it’s a bit weird to use a WordPress site as the point to glue together multiple ICS feeds to create a new one intended for use externally to the website, and since it’s easy in desktop/mobile calendar software to subscribe to multiple feeds and display them in one view, I’m not sure there’s a huge use case for combining them externally first.
It’s kind of a “MacGyver” approach that would probably work, but is not really the right tool for the job. (It’s hard for me to describe why without getting into a multi-paragraph digression on the stateless nature of HTTP, OS-level cron jobs, etc.)