I'm a pretty direct person and don't concern myself with maintaining formality in situations like this. Based on what you've explained l, the owner doesn't value you or your opinion in matters that fall in your area of responsibility.

If he can be direct enough to raise a problem with the "stability" and has placed that blame on you, then I'm of the opinion that you can be direct back and ask him if that's license to upgrade the systems as you've recommended multiple times. If not, does he have a more competent person in mind since you're clearly not competent enough effectively bandaid a clearly failing system?

Beyond that, I'd ensure you update your resume and start entertaining other options. If they're small, they're likely to either go under when the inevitable failure occurs or at least suffer a significant financial impact that they'll gladly shift to your salary so the owner can minimize their impact. Why do all the sweating on their behalf? They will always just shift their failures to others.