1) One can read a text with an eye toward the ways in which the narrator might be misinterpreting the situation and can hold in ones' head the possibility of that misinterpretation...while also making suggestions for how to grapple with the problem that the writer presents.
2) Taking as given the way OP presents their problem, you are correct that the co-founder is not acting in the best interests of the business. However, it is an unjustified leap to say that the co-founder does not care. There are a multitude of reasons why the co-founder could simultaneously care very deeply and be acting this way:
- The co-founder is mistaken on matters of fact about the problems.
- The co-founder is making a judgement call about maintaining product focus vs responding to customer requests...and making a mis-judgement
- The co-founder lacks skill at listening to and interpreting feedback.
- The co-founder is overcorrecting from making an opposite error previously in life/career.
- The co-founder is missing some other very important leadership skill.
Point is, it is possible for people to fail very badly and obnoxiously at something that they care a great deal about.
On second read, I think you might be using the phrase "does not care" in a way that isn't making sense to mean. Could you expand on what it means for someone to think they care about something but to not actually care about it?
 This is a skill. https://www.amazon.co.uk/Thanks-Feedback-Science-Receiving-W... is a good book on it.
This is a rather broad and not very actionable criticism. Can you get more specific feedback? For example, something of the form "In situation X, you did/said Y, and that led me to think Z" is more actionable. Also, it is useful to pay attention to the classification problems that your engineer faces. There is a sometimes a balance between being condescending by giving someone info they already know and not providing enough context for an explanation.
The book "Thanks for the Feedback"  is a good read and Erika Carlson's talk from Lead Developer UK is a good watch.
I think it has made me a better teammate, mentor, and mentee.
If you'd like to know more before buying it, check out
One book I've enjoyed which equips confidence is "Thanks for the Feedback." It's a book by Douglas Stone & Sheila Heen of the Harvard Negotiation Project.
Prepare to read one chapter every few days. It's a heavy read. Well written, it's an abundance of information: you may need to take time for brain rewiring as I did.
Thanks for the Feedback - http://www.amazon.com/Thanks-Feedback-Science-Receiving-Well...