In order to fulfill the basic functions of our service, the user hereby agrees to allow Xiaomi to collect, process and use personal information which shall include but not be limited to written threads, pictures, comments, replies in the Mi Community, and relevant data types listed in Xiaomi's Private Policy. By selecting "Agree", you agree to Xiaomi's Private Policy and Content Policy .
1: I could say some apps may not work correctly in pie (like apps cannot send notifications)
2: better notch support in pie
3: security patches!?!
4: some app are better supported in Android 9
5: some themes are no longer supported on newer miui versions
and other stuff ...
Newer versions have latest security patch and latest features, i recommend you to update to the latest version but still it's your choice i mean if you feel that your current version is doing a good job then you can stick to it