Having well sanded features doesn’t necessarily remove bugs directly drive conversion, but it gives the entire product an established professional “feel.
Even though your customers feel it, they won’t explicitly articulate it
Is there actually a competition where the UI could decide the winner?
I mean, it is probably very rare to have two products that have the same features, and the only difference is how nicely done they are. For example, MediaWiki is in my opinion 100x better than Confluence, but Confluence has some extra features that I don't care about but managers do (stuff like easily attaching PowerPoint presentations to web pages), so the managers decide that the company uses Confluence. And I keep silently screaming in frustration every time I lose a part of text during editing, or the links break when a typo in a page name is fixed (because there is no option to add a redirect), etc.
The extra feature often wins, when the salesman describes the product to the manager who makes the buying decision. That's why we have the "checkbox features" that most end users don't care about, but it allows the product to seem better in comparison. Feature creep is how stuff gets sold.
The situation of two featurewise identical products competing only on better UI would probably be highly unstable, even if it happened somehow. There are network effects, so if one product starts winning, most people will switch to that product because "that's what everyone else is using". The other product will be left without money to pay for development, and will go out of business. Afterwards, the winning product does not have to care about their UI anymore.
