I don't think xbox can realistically announce it all at once - that would be a sega move in terms of killing their user confidence stone cold over night.
Unlike Sega, MS have to consider the 30m GP subscribers and try to keep them on board to some extent. The stench of failure can close up a wallet, and while the hardware is dead, GP still generates income with most of those subs on the console. Kill the console with bad news, spook the users to other platforms, and those subs die too.
I'm already a little surprised at FH5 coming before they looked at Starfield, which was supposed to be multiplat before the Zeni acquisition. It would have been an easier message to start with those newer acquired studios before going to Forza, yet they've gone straight in for a core Xbox franchise.
I think they'll let this news bed in for a while - months maybe only weeks - and then announce the next one. Even these leaks / rumors are setting the scene and letting the user base come to acceptance.