That's like saying GTA reuses cars because they're the same style and name. Of course they are, it's the same area. It wouldn't make sense for them to replace all the cars. They HAVE been updated though, and there are several (At least 10 not counting the alien stuff) new models and way more customization available for most of them.
The guns... not even. All the previous guns are there, yes, but there's at least twice as many guns as there was before. And that's not counting the extra models (like the guitar case for the rocket launcher).
Spoilers below:
I didn't want to mention it before but I'm not surprised you didn't notice how little you spend in the city since you only played the beginning. It's about a 25 hour game depending on how much you unlock everything. I'm sure GTA5 is longer but hell, it's hard to fill time when you can move around so quickly. You go to places from Saints Row past, you go into a completely 2D world, you go into the 50's (as you saw at the beginning for a bit), you go to the "real world" several times, and many others. None of that is the city.
Anyway, I feel like your major beef with the game is that it's in the same "city". Yep, that's a thing. But considering it's been modified, you don't spend most of your time there, and the way you experience it has changed, it's hard to care. I don't think if it took place in Los Angeles or something that it'd suddenly be a better game.
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BTW, if your saves are deleting themselves, it's probably Steam Cloud. Steam Cloud can get really, really aggravating if it loses sync with your system. I've lost Skyrim saves before as well, through no fault of the game itself.