> There are no "minimum production values" if your game is good enough.
Which is just a tautology.
Hotline Miami - 2 core devs + 2 others;
Risk of Rain - 2 core devs + others;
Gunpoint - 1 dev (Tom Francis FTW, yay!)
Valdis Story - Dunno, but website says "we" and mentions a few different people in blogs;
Terraria - 2 core devs + others;
Vlambeer - 2 core devs + others.
It is very difficult, almost impossible for a single developer to make a competitive game now. Not actually impossible, but the dev needs to be almost preternaturally productive. It is much more common to find programmer + artist + others teams, as it became after the transition to 16-bit. Very common to have programmer + artist for pre-alpha prototyping, contracting additional programmers + additional artists + sound guys (as for several of the games you cite) + platform conversion teams + music, etc.
> pixel art using GameMaker, for example.
Pixel art can have high production values. Pixel art was still the main requirement for 16-bit games, well after the demise of the bedroom coder.
If you think that, say, Risk of Rain doesn't have many person years of production input, you're deluding yourself. Try making something equivalent yourself this year.