sandboxing is a good thing ,since most of windows usability issues are because of non-sandboxed apps (adware , viruses and bloating overtime) ,it's actually great for mobile and desktop platform .
you can stream to twitch and mod games , all of it is possible (they actually demoed modding in last Build conference) , the store also can have exe apps and they will run isolated (without an installer or uninstaller , just simple one click install and uninstall without leaving any junk)
here's more about how to put exe apps in the store , it's coming to the public with the next windows July update
windows today have a lot of issue and the store can solve most of them if microsoft was successful with it (especially adwares and viruses)
what is more important is that most people don't care about all the stuff you mentioned (overlays and twitch streaming and even modding) they want a solid gaming experience and UWP games are better at that,,things like online cheating mods are not possible with UWP , mods are possible when the developer allows it
the technical works is done for the store (with July update) ,now they need to go and get every major developer and publisher to adapt the store