I could see it being nice for software appliances. I spent many years working for a company that made an appliance (run this OVA/stick this pre-imaged box on your network), and they had this godawful mess of perl they used to orchestrate the box (e.g. updating a configuration file from the GUI and then restarting the sysvinit (and later systemd) service). I could see someone writing a system orchestrator in Guile that, rather than shelling out to systemctl, imports (or whatever it's called in Scheme) the service definition and directly starts the service using a function call, complete with error handling and all of the nice shit that you don't get by execing some binary.

libsystemd exists for systemd which lets you have some of the same benefits, but it's a C library which doesn't integrate nicely with all languages. I remember not liking any of the Python wrappers I tried, even though Python generally does a great job interfacing with C.

Like how the average computer user is never going to [...] install Firefox or whatever.

Not right know but in 2005-2010 (or something like that), the average user was installing firefox because IE was so bad. It used to be at 80% market share IIRC.