And yet evidentially, they aren't so very fragile. Clearly there's something about coops that make them viable businesses, even if that goes against our intuition for what viable businesses ought to look like.
I think part of it is that workers typically have very good insights into the immediate needs of a business. You bring up technical debt as an example of the dangers that businesses face, but technical debt is famously a problem that is noticed initially by the workers, and only later by the business as a whole (as velocity falls and new features start becoming painfully slow to implement). The companies that handle technical debt the best are typically the ones where developers are given the freedom and flexibility to make decisions regarding paying off that debt.
Similarly, you might expect that when the small problems in a restaurant show up, it's the workers who will notice them first. And the point of a coop is that those workers then have both the motivation (it's their business) and the power (it's their business) to fix those problems immediately, rather than being forced into handling less relevant tasks by a management that doesn't have full visibility of the problem.
Fwiw, I'm a fan of the idea of worker coops, but I don't think it's the only setup that works, nor necessarily the best setup in ever case. But worker democracy as a whole - the idea that workers in a company must have a right to contribute to the decisions of that company - seems both moral, and also practically very useful. If you want a good overview on this topic, I recommend the YouTuber Unlearning Economics, who has a whole video on worker democracy and what different forms of it look like (and what the costs and benefits of those forms are):
https://youtu.be/yZHYiz60R5Q?si=t4Q9qQLc9K7wsIx_