> we should never just have one product
I believe in never keeping all your eggs in one basket, which includes never having a single source of revenue. It's better to have multiple revenue streams and, if possible, to have them different enough that if conditions turn bad for one, it won't negatively affect the others as much. Even better, if you can swing it, is to have complementary products such that adverse conditions for one are favorable conditions for the other.
Diversification is protection. Even if your single product is wildly successful, the world is a fickle place and a successful product can stop being successful faster than you think.
I have seen single-product companies succeed for decades, though. What they do instead of having multiple products is to have multiple variations of their product, each sold in a different market segment.
