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.
Consider a personal relationship. You can say you love someone. You can want to love them. You can even convince yourself that you do. But you may not, in fact, love that person, and your actions will bear the betrayal.
What I'm talking about when I say "care" is caring about the business in and of itself. That's a very different thing than caring about what you get from the business. My interpretation of the OP is that the co-founder's behavior indicates that they care about what they get from the business but not the business itself. Someone who truly cares about the business itself doesn't act in ways that harm it unless they absolutely have to.
Businesses are full of people at every level who profess to "care very deeply" but don't, or who genuinely care about things that are immaterial or even detrimental to the business's success. So when I say that people like that "don't care," I am not saying that they don't care about anything or that they have no feelings of caring.
I am talking about the CEO who "cares deeply" but refuses to fire poisonous employees or make other difficult but necessary decisions, the dev who "cares" about his work but not enough to overcome procrastination, the HR people who "care deeply" about employees but actually fear them, the managers who "care" about their direct reports but treat them like shit, and so on.
I use "I care about X" to be a statement about the internal state. "I care" is a description of emotions and motivations. So which meaning is correct?
Well... its semantics, so no "correct" in the same way that there is a correct measurement of the gravitational constant of earth at sea level above average-density crust. I like to say "All models are wrong; some models are useful", and the purpose of words is to communicate. So which definition is more useful? I assert that it depends on your time horizon and your ability to convey relevant insights.
In a short time horizon where OP doesn't know what this guy's problem is, your definition is more useful for deciding how OP should act. OP has little way to open a heart-to-heart conversation where the cofounder can open up with his worries or commit to starting on a path to change his habits. OP probably doesn't have time to wait for the cofounder to change his habits unless they sit down and rigorously design those habits and the communication protocols to reinforce them -- something I've only seen from my wife and I. In a short time horizon, OP has to deal with his or her cofounder as-is.
So why am I still blathering on?
Because reading "he doesn't care" struck a nerve. I've been "the dev (or student) who 'cares' about his work but not enough to overcome procrastination" for multiple chunks of time from 2004 into early 2017. What changed wasn't my level of concern for productivity but instead several types of growth:
- Getting access to tools that allowed me to block distractions.
- Developing good habits of diet, exercise, and especially sleep (and escaping from a uni culture which said "sleep is for the weak")
- Learning skills of breaking down projects, insisting on understanding the 'why' of projects, asking well-structured questions, and planning out steps to execute.
- And 14 months ago, learning what discipline even actually was and how to build that muscle.
This growth all happened over the course of a decade of trying to debug myself which included 4 PIPs and 3 firings, so quite a long time horizon and empirically a longer time horizon than anyone but my wife could reasonably have had the patience for. If you had told me during this time that I did not care about my work, I would have been internally seething with anger, but not even known how to counterargue. So just so you know, concluding that about someone can be rather inflammatory to them.
By the way, thank you for answering me. Writing that all out was really useful for me. If you'll humour me again, I'm curious: What are some ways to tell that you don't love another person? What is love? When I've asked about this sort of thing before, I've just gotten the response " Baby don't hurt me; Don't hurt me--no more ", which is unhelpful.