Every time Tesla introduces UI "improvements", it's been two steps forward, one step back.
With V9, we've lost the ability to have two non-NAV apps displayed at the same time, and the ability to put a non-NAV app at the top of the display. But, we now have the ability to control the height of some of the apps, so they are no longer required to take up 1/2 of the display.
Before getting too concerned about this, recommend you try the new UI for a few weeks - and then make requests to Tesla for improvements.
What's more important than the split screen changes is the usability of the interface. For typical operations, does it take more or fewer screen touches to perform the operation? If it takes less, then V9 is an improvement. If it takes more, then Tesla should look at additional improvements to V9 to make it easier to use.
Since the console display is intended to be used by a driver - while the vehicle is in motion, if V9 requires more screen interactions than V8, for functions used while the vehicle is in motion (such as climate/media player controls) - then fixing that should be a high priority (and perhaps more important than the split screen changes).