Compared to....what? RPGs? Sure. However in the open world action genre San Andreas is one of the more notable games in this regard. Certainly way more gameplay depth and complexity to GTAV/RDR2 and the majority of the modern shit.
If you just went back to GTA3 and gave up I could understand, it is a bit barebones, never thought much of it back in the day and still don't, but with each sequel they got deeper and more complex culminating with San Andreas and then it was all decline from there. That said, San Andreas is by no means this deep complex highly engaging game, and the combat really needs a polish up. but I struggle to think of much of anything better in the open word action genre (unless you bring RPG into the mix). Stalker and The Saboteur would be worthy competitors I suppose. I like a nu Far Cry or two but it has a lot of popamole DNA in it.
Also, golden GTA trilogy has a massive modding community...the core of the games is great in concept and mostly good in execution, but in need of updates to be truly great. You should delve into that.
Dude, I even made my own mods for GTA SA. Don't just assume stuff. It's a wonderful game, definitely the most complete game they made and the best showcase of Rockstar's capabilities at their peak. Then again, with all the things you can do in that game, there's not much you can really sink your teeth into if you've done it all before (to their credit, I remember most of it after ~20 years).
Why do I say the gameplay is mostly shallow? Most missions feel like tutorials (the Rockstar design philosophy) or are a variation of go somewhere, watch cutscene, then have a shootout or a car chase. Compare with Mercenaries, which came out around the same time (I imagine it resembles Saboteur, which I haven't played). That game gives you total freedom in how to approach any mission. You can use stealth, action or hire outside help, come at he target from any direction, etc.
Mafia, which didn't manage to avoid repetition, also gave you a lot more freedom and in general required more brainpower and initiative, being more convincing in putting you in the shoes of a gangster.
Unless I'm on a GTA nostalgia streak, I know which games I'd rather replay. For me the most important factor in replay value is allowing me to discover the game again by doings things differently, or being forced to rethink something which I thought I knew.
Again, GTA SA is an amazing game, but at some point Rockstar should've updated their design philosophy. They didn't, which means the remakes will be more of the same with some expanded functionality and pretty graphics (which I'd rather do without). That's if they don't fuck it up in typical current year fashion. I'm not saying I won't play it, but it's definitely a wait and see scenario.