I believe the chrome steel collar, this collet type piece that it grips, and the cut out area of the steerer they're sitting on are how the stem is held tight on this bike. The aluminum collet piece may be there so it's an aluminum and not a steel thing gripping the steerer threads. So, it may not be threaded as such, but is pushing up against threads and ...
If your Nishiki Meridian:
meets your riding needs
then it is worth getting into riding condition - especially if you do the work yourself.
Air the tires, do a safety check, and ride it around enough to get a feel for how it rides. If you like it then it's worth fixing. If you don't like it someone else will.
Type of riding
It's categorized as a ...
That's an awesome bike - would be called a MTB back in the day, but now we'd call it a rigid MTB.
You probably have to replace the brake pads (the rubber blocks, 4 in total) because they go hard with age/ozone/UV and don't brake as well. That style of brake is called a cantilever and while less-common now, they can still work perfectly well when tweaked ...