It seems like all other comments have arrived at the same conclusion - This is a downgrade. This is not a new "feature" to streamline anything; this is a retroactive limitation. If users are trying to open Desktop and RemoteApps at the same time causing issues, this is a training and/or administration issue, not an issue Microsoft needs to solve. For years, Microsoft products have succeeded or failed depending on how SysAdmins configure things, and after all this time, now you guys try and put some guardrails to stop from issues happening? Come on...
This needs some serious reconsideration. More environments than not, users need to be in the same host pool but have the option to choose Desktop or RemoteApps. There are times where even the same user may bounce between one and the other (by properly logging off, preventing duplicate sessions) depending on which app they need.
As a real-world example, an improvement that is actually an improvement was the recently added support for OneDrive with RemoteApps (thank you for this!). I have multiple environments where the users only use Desktop because they need OneDrive with their other application for SharePoint integration. If I want to test the new OneDrive RemoteApp support with some users, rather than giving them access to RemoteApps alongside Desktop so they can use both for testing, I now need to restrict them to just RemoteApps, and if they run into issues with testing, they can't just switch back on their own. They instead have to call me, explain the test is not working, and I need to switch their App Groups so they can get back to work. This is a royal waste of time for the test users and me.
Give us our freedom back.