I can’t think of a single other video player competing on macOS UX adherence over the last 20 years, source/name a couple of these many others so we can see how they died. Because it would die.
Yes, if IINA is no longer modern, it will cease being modern.
If IINA doesn’t adhere to modern macOS UX standards, but advertises that it does, there will be a compelling reason not to use it.
In general, macOS apps require constant updates every year in order to run on new versions, period. The idea that an app that competes on UX would continue implementing updates solely in the non ui realm and as a result fall out of current macOS UX practices year after year is unlikely, to say the least. It’ll just die normally.