Something I found very amusing: the average life expectancy was probably about 20, because 21's the absolute maximum and there would have been significant deaths from failing infrastructure. This means that under the UN's current criteria, the Human Development Index of this society is negative.
Why would a force of practical assassins need a weapon with a stun setting? A Sandman's job is to kill Runners so why would they ever need to NOT shoot to kill?
Fridge Brilliance: Considering Jessica's character demonstrates there is an organized Runner's movement, a stun setting would be useful if you'd like to interrogate someone or use them as bait to catch their fellow subversives. It's also a nod to the books where a Sandman's pistol had several settings, including a "tangler" for live capture. Sandman were not just for killing Runners - they were the society's law enforcement. Unless you were on Last Day and trying to run, they were no more or less a threat than an average beat cop.
At the end of Logan's Run, the evil computer is blown up and all the people set free! Everyone goes outdoors and sees the sky and the trees. This is an entire population that has been given free food, water, shelter, heat, and clothes for their entire lives. Now they're in the wilderness with no knowledge, tools, or even clothing heavier than lingerie. They're all going to die. This probably also happened every time Captain Kirk saved a population from a controlling computer.
Stealthily acknowledged: the credits imply their doom by running blended shots of the doomed people flying over the Carousel over shots of the burned-out remains of the city. At least the people Kirk rescued apparently still had their cities and machinery intact and a little Federation aid.
The books ran a similar scenario; while there was certainly a social collapse and reversion to barbarism (even more so), humanity muddled through.
On a more optimistic note, the old man must have some sort of survival skills and tools to have lived outside for that long though (quite healthily too, for someone looking like in their 60s). People can learn from him. Not a lot of hope but still quite some hope.
There is also the fact that they all can read, and books have survived whatever catastrophe wiped out the previous civilization, so they can learn from them too.
And of course there's the not-insignificant possibility that some other humans are still out there and can teach them how to survive. After all, that old man had to come from somewhere...
In the movie, Logan's destruction of a support column and overload of the Master Computer shouldn't have cause that kind of cascade failure through the whole city. However, remember that no one likely knew anything about maintaining the city aside from cosmetic appearance.
In a meta sense, the timing of the movie (1976) and the age-upgrade from 21 to 30 made it about the early 'mid-life crisis' of the Baby Boomers. It was cathartic for the generation that said 'Don't trust anyone over 30' in the year their oldest members turned 30. It ends up subverting the phrase, by saying 'being over 30 is fine.'
A bit of Rem brilliance mixed with Fridge Horror. When Logan and Jess find him, he's been slaving away maintaining servant bots for a pleasure palace where all the humans they were designed to serve are long dead. Rem has a lot more sentience than the robots he's maintaining (so no pleasant conversation or AI society), but he's still Three Laws-Compliant. No humans were around to protect, or give orders, and he couldn't self-destruct or allow himself to decay into non-functionality due to Law #3. It's centuries of boredom without the means to escape it. Then along comes two humans who admit to being fugitives. They seem to be pleasant enough, but they have a little trouble with the "orders" part. So, Rem essentially tricked them into Law #2 by giving an order (by inviting him to join them). If they find their Sanctuary, then he's surrounded by humans and happy to be back at work. If they run into trouble, then Laws #1 and #3 would kick in and permanent non-functionality would be preferable to more centuries of boredom.