Not Evaluated Description
Nowadays, macOS have a new release every year, Windows 10 every half year?
For example, macOS 10.14 update need very powerful graphics card, so I assume lots of current users will keep using 10.13 if their machines can't upgrade to 10.14.
And if we want to support 10.14, we need to use some new or latest API, which are not available for some old versions, then it means the users which depend on them are "deprecated* too.
Similar story, we have lots of users are still using Windows 7. If we still keep them in latest code, perhaps we can't migrate to the newest features very easily.
If we could split(or decouple) our current code base to two part, Qt libs + platform related plugins(platform + style + perhaps more). That's what QPA designed for.
At least I think it will help a lots for those style things on old operation systems.