Let's talk about games with sequels or spin-offs that overshadowed those that came before it.
Whenever you say 'Unreal,' people think you're talking about Unreal Engine or Unreal Tournament. Just about nobody thinks of the single-player game UT spun off from. While UT is pretty rad, it would've been really cool to see more atmospheric and adventure-centric type stuff like Unreal. I guess there was Unreal 2, but that game was really different and mediocre as hell.
Unreal 2 followed more of the Halo vein. The basic gist of it is that I doubt most of the devs who worked on Unreal 1 at the time strictly wanted to make a singleplayer game. There was a lot of work done on the multiplayer and while the netcode was shit it did come with bots.
After Unreal Tournament sold an enormous amount it makes sense they chose to focus on that. While there is a lot of demand for another Unreal game it's pretty clear given how bad Unreal 2 was, just in terms of premise and execution. That Digital Extremes really had no interest in making another singleplayer game. And that Unreal 2 was just made, just because.
It's really a good example of where wanting a sequel to the game isn't necessarily a good thing if the dev has no real idea for one. It's often better to just let the original stay as it's own thing. Had Unreal 1 had like 5 mediocre sequels do you think anyone would've still looked upon it as this quaint forgotten FPS?
Wolfenstein and Duke Nukem both count.
Hell, I bet there are people on this very board that think Wolfenstein 3D was the first in the series.
Red Alert.
How many people even know Galaga is a sequel, seriously?
To be fair it was by a different developer and was only loosely based on the original two games. It was really a sequel in name only. The same goes for Duke 3D since in it's alpha state it was originally more dystopian and bleak in tone but then turned into a farcical game later
Mass Effect 2 overshadowed the original, despite being much more shallow and destroying the setting.
Also, every numbered Elder Scrolls game.
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None of those games are overshadowed by sequels though.