I haven't read this elsewhere so it may be non-validated but you shouldn't imagine customers of a product as entirely captured nor cohesive. Inside of the customer pool are going to be people who are displeased with the product for sometimes disparate reasons but not enough to find a different product because this one is "good enough".

To make this example very simple, a product may produce orange widgets. It's good enough for most people but one group wishes it made a more yellow widget while another group wishes it made a more red widget. The orange widget isn't the ideal case, it's a compromise to serve the most people.

If someone came in and made a product that produces very yellow widgets and focuses on the yellow widget market, you could serve those people very well despite being having a smaller pool.

In other words, your product doesn't need to go toe-to-toe with the orange widget product that has a revenue of $1B, you only need to pick off the yellow widget people to get a revenue of, lets say, $10M and you are eating very well.