The finest contribution of AIX to Unix, copied (with varying attempts at improvements) by its 90s competitors and eventually into Linux, was its jounal filesystem.
The order of magnitude improvements to fsck on bootup and overall filesytem reliability were non trivial.
SMIT was abit too non-unixy to catch on but I've always wondered in the back of my mind whether it influenced systemd...
As a sysadmin in a mixed shop in the early 1990s I found that I could do just fine with traditional CLI and scripting tasks without using HP-UX's SAM or Sun's Solstice tools, but AIX's SMIT and smitty really got in the way of that sort of cheap and dirty automation. I understood the purpose of all those tools, but AIX pretty well made SMIT and smitty obligatory and that did not sit well with me back then. In the context of what you've said comparing SMIT and systemd, I would agree based on my experiences of way back then.
Smitty didn't get in the way of scripting, you could just craft up what you wanted and was F9 iirc you could press and see your options all as they would execute in the command line. So could just go thru smitty to set what you wanted and instead of running it, just crib the command line sequence and pop into your script without even having to learn AIX nuances.
I liked smitty for the <ESC>4 that would show the script/command that smitty would use. I used this feature a lot in the beginning to learn the IBMisms of AIX.
The order of magnitude improvements to fsck on bootup and overall filesytem reliability were non trivial.
SMIT was abit too non-unixy to catch on but I've always wondered in the back of my mind whether it influenced systemd...