The one thing you can learn from sysv init isnthat asking devs to pitncode into their programs or into starter scripts does not work. They will not bother: Those will notmworkmcross platform.

So you need to cebtralize that task. You can either write a wrapper program that sandboxes starts applications in a sandbox or do that whereever the programs as are started anyway.

A separate sandboxing app that starts services complicates configuration: You basically need to configure two things the starter and the service. On the up-side you have the sandboxing code separate. Merging the sandboxing into the program starting the service makes configuration simple but adds moremcode into the the starter program.

So it is basically a decision on what you value more. Systemd decided to favor simpler configuration. The cost for adding the sandboxing is small anyway: It's all Linux kernel functionality that does need a bit of configuration to get rolling, with much of that code being in the systemd-init anyway: It uses similar functionality to actually separate the processes it starts from each other to avoid getting confused by programs restarted and thusnchanging PIDs -- something still a thing in many other inits.

I am convinced that making sandboxing easy does a lot formits adoption. No admin will change the entire startup configuration to add a sandboxing wrapper around the actual service. It is way more likely for them to drop in a override file with a couple of lines and without any problems when upstream changes command line options.