But we don't get that in modern software. We get "Something went wrong :(".
Complex problems are not easily made simple or easily reasoned about. Something like systemd (or any non trivial piece of linux) could never be made simple or, with your hands down in the guts of it, easily understood.
Breaking systemd's functionality into a million separate pieces wouldn't make the complexity go away, it would necessarily increase it.
I don't know what the solution is, but "making things simpler and easier to understand" is a bit hollow. Its problem space was never simple or easy to understand (unless you think systemd just runs some things at startup).
With older _nix setups that was fully possible because daemons were pretty much (i am likely to get dogpiled on this) a user process without a shell to send output to.
With systemd on the other hand the state inside a systemd-managed unit, and the state outside of said "black box" may be wildly different. And to debug the person doing so have to attempt to maintain a massive mental image of state.
Frankly the kernel may well be the closest approximation, and it may well be better instrumented at this point. Never mind that building a second kernel in userspace is not a kudos point.