The fact that you compare iA Writer with Scrivener is pretty funny.
The first is an overpriced minimalist lifestyle app. It has some value, but definitely not what they sell it for. The app is a rather simple development with a low number of features, most of it can be found in many equivalent apps, for free or close to that.
The second is an actual tool that handles a lot of complexity inherent in writing large bodies of work with a lot of features. They sell it for barely more money than iA even though the development and support cost must be much higher.
You complaining about old looks and the niche features of Scrivener illustrate exactly what is wrong and rotten about the Apple platforms in general: the cult of minimalist bullshit that results in basically useless software that only achieve one thing, looking good.
Even macOS has largely suffered from the trend it is extremely appalling but since this is a trend that shows up in Apple owns software, I don't think there is much hope to be had.
The point is that Scrivener is a mostly done work and there is not much left to do, change for the sake of change doesn't make any sense even if it might please your sense for new looks or whatever.
The design trends are irrelevant for what is a tool, and just like a good screwdriver that looks old is still good 20 years later, a good software that looks old is still good 20 years later provided you are able to run it.
And this is exactly the point of owning software. The developers won't be responsible for it running well on the latest system, but it will run with the original setup as long as you can keep it or recreate it one way or another. That is the only thing asked and it is fair.
While sometimes there are meaningful improvements to some software, it is mostly a sales tactic, especially used by Apple to market questionnable changes and features that end up costing more than they are worth very often (time and money).
In fact, I would argue that Apple has created this whole update argument nonsense in order to force people to buy new hardware and make it easier and cheaper for them to maintain their systems. I think it is not right that macOS system have such low support time, especially with the embedded browser when you paid so much for it. Being able to update is not a good argument since not only do they change the performance profile of what you bought in many ways but sometimes they even remove features or break compatibility on purpose.
At least back then you could downgrade easily but now with Apple Silicon it's almost the same bullshit as iOS devices.
Subscription only makes sense if you have running cost (server software, or infrastructure bits) but is mostly nonsense for most consumers' use cases. You need to purposefully introduce breaking changes or shallow superficial reason for it to exist. Which is exactly what Apple is doing, because money...