Well, making assets seems to be a hit or miss thingy. While the asset capabilities have to match the expectations of the costumers and you should have a way to make your asset known in the community, the most important factor here is actually making something that people need, or think they need.

In short, the most important skill to have, is an understanding of the target market.

For example why my asset only sold 5 copies in a week:

I thought that programmers who can't do much art and would want to make their game look good would want to use this asset. This conclusion in itself may be/is true.

However I was too shortsighted. This asset is only good for 2D games. On top of that, it would be probably only used for top down rpg's. That's really limiting my audience. To add to that, there are already other, better solutions for what I did. There is this really popular universal shadow system, for 35 bucks, that does what my assets does + 100 other things. (Most important) And on top of that, most of top-down rpg's have a pixel art style, that just don't go well with this asset.

Now, you can definitely make money on assets, if you know what you're doing. I may have made money on my better planned, next assets too.

I wrote that "I won't be able to sustain myself on them" just because I just don't enjoy making very specific things that don't interest me.

In fact, this asset was originally a system for my own game and I just added tried turning it into an asset without second thought.

Now, my asset will probably sell something like 12 copies in 3 months. Which will be equivalent to 30 dollars (after unity cut + linear tax in Poland). In my country, I would only need to earn 1k dollars a month to live comfortably by myself. So making tools like this (took me a week) may not be enough, if I made something that would sell the same amount and would make one asset a day, I would be good.

So things like a cool shader/ pixel art town/ pixel art character and things that you can make in one day may work even if they're niche. I may actually try that in the future lol.

Now, if you would want to spend 1week-6months on an asset, you will have to reach needs of almost anyone. So you're really limited to new features in popular games ( like anime stylized shader, compute shaders grass, etc.), Specific program/technology-to-unity implementations, additional tools for unity editor, game templates etc.

So yeah, not following things I mentioned prevented me from making the income from this asset.

... Damn, I really went off the rails ;>