logind is just one example
Does nothing a script can't do
Do you really think it is a serious argument is that you could re-implement "logind" as a bash script? We are talking serious hardcore system stuff here, which is why no-one have made an alternative to "logind" or "ConsoleKit" despite upstream projects have pleaded for such a program for several years.
Systemd doesn't even fucking use capabilities, just cgroups. Which we could use before systemd. Systemd manages permissions in lieu of using capabilities, e.g. apparmor or selinux.
You are seriously misinformed on how systemd works and what it can do:
It uses kernel namespaces and capabilities to protect the system; this is on top of SELinux etc.
Here is a general overview:
Here are some of the config options for the daemons you can use. See "CapabilityBoundingSet=" for one way of using kernel "capabilities":
There are so many freaking cool security features in systemd. As time goes by, developers, distro maintainer, and systemd administrators, can add more and more options to the running processes, like "NoNewPrivileges=" to prevent privilege escalation, or "ProtectHome=" to prevent malware and exploited processes from stealing info from /home, even if they otherwise had permission to read in home.
All this great new stuff can be turned on and used by adding a simple keyword to a structured text file. As time goes by, systemd distros will become ever more hardened.
It only runs one process as PID1, the daemon "systemd" which is rather small. This daemon however, is capable of "talking" with with several other processes, which gives it many advantages,
This is making init do stuff it doesn't need to do, which makes it more complex, which makes it more fragile. You should not need a detailed explanation to understand why this is a bad thing.
Well, it does need to be handled somewhere; if you want features, you will get complexity, it is that simple. But as explained, the features and complexity isn't running in PID1; PID1 (systemd) is just a hub for relaying those features to other processes.
I really think so much of the systemd opponents talk about "Unix way" and "PID1" should be simple, is hand waving to gloss over the fact that the non-systemd distros have no feature parity with systemd to speak of; SysVinit is crude and no one in their right mind would design a init system these days with executable config files. Service configuration files should be non-executable text only.
General and vague criticism against systemd really doesn't convince anybody. Anyway, the Linux community have spoken with a large majority of Linux distros using systemd in the future.
If SysVinit systems really have all the features of systemd, just much better because they are simpler, you would expect a "SysVinit" boom in the future with lots of developers and users.
Personally, I think the systemd opponents are too concerned with negative campaigns against systemd, that they entirely forget to code any alternatives, so I predict ever more distros like Slackware abandoning script based init systems; they simply don't have an alternative.