This doesn't actually provide the benefit of application isolation though; if the software is malicious or vulnerable the as-user component could be as well. Remember that the biggest use case for application isolation is untrusted applications. Essentially any setuid-based approach to isolation requires a trusted developer using very good practices to remain secure, and that's why it's faded away.

What's insecure about setuid if the setuid user isn't a privileged user? For example, a setuid-nobody program, shouldn't be any more insecure than a systemd service spawned as User=nobody, no?

like I sayed: hacks