Or a future update starts to use those reserved high bits. Some of the current encoding space restrictions are just that Unicode didn't reserve enough high bit

Or a future update starts to use those reserved high bits. Some of the current encoding space restrictions are just that Unicode didn't reserve enough high bit surrogates for UTF-16 to extend indefinitely. (UTF-8 can in theory. UTF-32 has options, including some of these still reserved in UTF-8 but currently unused high byte codepoints, UTF-16 is accidentally stuck, for now.) Sure, it is unlikely that we'll see another Unicode plane extension in our lifetimes, but many of the people that bet on UCS-2 when that looked like it covered everything and are consequentially now stuck with the somewhat broken UTF-16 thought the same thing.