Yeah the architecture for electron is absurd, but it's important to not relegate UI flexibility to mere aesthetics. For most of my career, I was a back-end web developer, but more recently I've done a lot of interface design after getting some formal education in it. The overwhelming majority of developers I've worked and interacted with conflate aesthetic and interface usability. Heck, even I did before I really started digging into it professionally. I think it's because applications that have experienced designers make a good, usable interfaces will also likely hire visual designers to do aesthetic/branding work, and especially in waterfall environments, developers get it all handed to them as a "design." And for many reasons I will (uncharacteristically) not rehash here, FOSS lacks both.

However, a good interface and a pretty interface are not the same thing-- both are communication mediums that communicate through the software interface, but visual/branding/identity designers communicate things to the user about the brand as a marketing device, and interface designers figure out how to communicate the software's features, status, output, etc. with the greatest efficiency and reduce unnecessary cognitive overhead. Branding and identity is a very specialized form of design that's usually done by design houses-- even huge companies with big teams of designers often contract this work out to specialists. They might go so far as to recommend certain animations for interaction, but you don't want them designing your interface. In small companies, the designer will probably have to implement their design to conform to the design document, but they're using tools like gestalt, alignment, color grouping and type to create information hierarchies, existing expectations for layout and functionality, etc. that tell the user what they need to know as effectively as possible, and how to act on that in the ways they need to.

A good example of the power of interface design is in many dark patterns. You can simply have a plain system-standard dialog box asking if a user consents to some creepy analytics that nobody really wants, but instead of "OK" and "Cancel" in their normal spots, put "Off" in bold letters where "Ok" would normally be, and "Consent" in non-bold letters where "Cancel" would normally be, and I'll bet you at least 60% of users would choose "Consent" having only skimmed the familiar pattern. That experience isn't branded or styled in any way-- it solely uses juxtaposition, pattern expectations, and context to influence users behavior.

When you've got an inflexible, counterintuitive UI kit that developers must fight with to get the results the interface designer carefully put together, you hurt the usability of that tool for end users a hell of a lot more than mediocre performance does. This is very counterintuitive for most developers because of the curse of expertise. We have a working mental model of how software works on the back end and consider the interface a tool to expose that functionality to the user. To users, the interface is the software, and if you're design is more informed by the way the software works under the hood than the way a nontechnical user thinks about solving the problem they're trying to solve, it's going to be very frustrating for everyone who isn't a developer. Developers like to talk about marketing as the primary reason commercial software is king, and it's definitely a factor, but developers aren't magically immune to marketing, and you can't get much more compelling than "Free." To most users, the frustration of dealing with interfaces designed by and (inadvertently) for developers is worse than paying for software--- hence FOSS nearly exclusively being adopted by technical people.