> companies with only one product, aka startups.

from the top of my head:

http://renoise.com/ https://www.gpsoft.com.au/ https://www.ultraedit.com/

Incidentally, they're the best in their respective fields I know of. They don't make products, they don't make product, they make and improve these particular applications. I would LOVE them to expand and make, say, clothing and hair gel with that same intelligent, no-nonsense craftsmanship approach. Not to mention operating systems and search engines. For now I'll dream of Directory Opus for Linux and not even get that.

I don't think this necessarily follows. There are established companies with long track records who specialise in one area.