I agree with this. I use TV for watching streaming channels (roku) and live TV (samsung). At the moment I have to turn off activity a to turn on activity b. I am about to put mine into "production" use today (I've upgraded my roku and thought it would be good to get rid of my harmony remote), but the family acceptance factor on having to turn off and on just to switch between TV and streaming (and back) is going to be annoying.. I can feel the complaints coming already!
I've seen some comments about input switching and that really doesn't work for this situation, for TV the remote is "mapped" to the Samsung stuff and streaming the remote is "mapped" for roku.
I really hope activity switching is coming soon! Harmony does it and it didn't really know the state of devices as sometimes they would fall out of sync, for example, it would think the soundbar was on when it was off and you'd have to manually turn it on or off to get it back in sync. I don't think you need to do anything complex, just work on "best guess" state, rather than trying to work out if a device is actually on.