Yes this. In a perfect world, you have a perfect design and lots of growth. In the real world, sometimes you have a lot more of one than the other. Trust me when I say you would much rather be running an app with 100k users and scaling problems than an app with 3 users that can handle 1 million users.
That said, what are you doing to fix the root problems? I bet they could use someone to take ownship of the large issues. Come up with a 1 year plan to resolve the issues. Maybe 6 bigger projects of 1-3 months each. Present them to the owners, and try to get buy in to do them over the next year. I bet you could work 50% on firefighting, and 50% on making big changes - and get the app to a good place in a year. I run a tiny startup, but I would love any employee that did that ;)
A startup with a product users love but creaking infrastructure is better off than one with excellent infrastructure and a product that's a dog.
Startups with both a product users love and excellent infrastructure? A lot rarer than you might think, especially at this stage.