also you are wrong about build , they have done their bit and updated the api , the devs of the apps you are using now have to enable it in their apps from what I am reading ;-)
You're on the right track here. There are two main reasons that the resuming screen will be visible less and less.
The first is the OS handles memory differently so memory efficient apps don't have to be suspended as often. There's no way around it for apps that consume a lot of memories so when memory is needed, those high memory apps will have to be suspended to make room.
The second is that WinRT apps (the new run time used in Windows and now WP8.1) is significantly more efficient with memory usage in normal usage. It's even more so efficient when handling large lists. This is why apps that have large lists with complex data like lots of info and images start to bog down and need to be resumed often in Silverlight apps (WP8 and earlier). Silverlight apps are just not that good at handling large lists and complex UI.
So to sum up, the resuming screen will be appearing less and less as the following take place:
-Apps are written efficiently from the start by devs that know what they're doing.
-Apps are running on WP8.1 and later.
-Apps are written in WinRT rather than Silverlight.