Earlier quoted context omitted.
Is your claim "for most companies the quality is already sufficient, and so it's not the limiting factor (even though it would be if the quality was insufficient)"? Or literally "the TAM matters, your app's quality doesn't"?
The prior. Now-a-days, the majority of developers can build a quality app since infrastructure and tooling (heroku etc) are widely available, that you can paper over most completely horrible / unacceptable experiences by limiting features early on. If you pick a huge market and have simply 1-2 compelling features, you can go from nothing to something within 90 days typically, and then use that to become unstoppable.
Two counterpoints: - Shopify came in and dominated Magento and others via a much better quality product (later they built network effects with app store, but originally it was just an easier platform) - Slack came into a workplace communication space where there were (arguably) free alternatives in IRC and others and won with a better, simpler product.
I think product quality is extremely important in determining success. I think it was less important 10-15 years ago as the web was establishing itself, but today consumers expect a polished, feature-complete experience.