IINA is born to be a modern macOS application, from its framework to the user interface. It adopts the post-Yosemite design language of macOS and keeps up the pace of new technologies like Force Touch, Touch Bar, and Picture-in-Picture.
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.