> Eye candy is definitely the appeal of a commercial app.
First of all, I don't think "eye candy" is a fair way to characterize UI/UX changes as they often make the software easier/faster/more pleasant to use.[0] Second, look and feel is important for adoption; the fact that the app is "technical" does not matter. Regardless of your audience, all else being equal, an app with a polished UI will be perceived as better than one with a dated design. Third, what makes K9 more "technical" or "sophisticated" than any other email client, to the point that you think it should be treated differently?
[0]: Yes, I'm aware that it can go the other way, but K9 is a great example of an app that could use a UX makeover.
FOSS apps and software were not optimized for adoption, but as a healthy alternative to further an ecosystem of open development and continue progress when others halt. The idea was for the next generations of software to be able to take the same open development and further it with various appeal to differing subsets of the masses. This includes those who want pretty apps. Or those who want UIs optimized for particularly unique workflows.
K9 is an example of an app that needs to update their UI. But it is also an app that needs to take its great set of features and innovate a little further on them. Commercial apps optimize for eye appeal, because those are the lowest of the hanging fruits, They also skew the design opinion towards the generic and easy adoption, which is why the more technical first-adopters get upset at every UI change.
Users with more complex needs are using K9. And yes, I am aware other apps are catching up.
> FOSS apps and software were not optimized for adoption, but as a healthy alternative to further an ecosystem of open development and continue progress when others halt.
I don't see why you can't have both? It seems silly to say that FOSS as a category is not optimized for adoption because there are plenty of widely adopted counterexamples like Linux and Firefox. And in the case of a mobile email client—something that a huge portion of the population uses—why can't/shouldn't it be a daily driver for non-technical users?
> K9 is an example of an app that needs to update their UI. But it is also an app that needs to take its great set of features and innovate a little further on them.
Unless you have unlimited resources, build for a single audience. And that means you are making your app opinionated to the needs of a group. So no, you can't do both.
However, Foss means Others can build the app differently if they wish.
First of all, I don't think "eye candy" is a fair way to characterize UI/UX changes as they often make the software easier/faster/more pleasant to use.[0] Second, look and feel is important for adoption; the fact that the app is "technical" does not matter. Regardless of your audience, all else being equal, an app with a polished UI will be perceived as better than one with a dated design. Third, what makes K9 more "technical" or "sophisticated" than any other email client, to the point that you think it should be treated differently?
[0]: Yes, I'm aware that it can go the other way, but K9 is a great example of an app that could use a UX makeover.