TLDR: If you could browse all streaming content through your devices UI, the argument against non-linear services will be over. Someone will probably tell me it already exists.
Full version: I think the one piece that is missing before streaming from non-linear tv (including YouTubeTV and other apps that stream traditional linear channels) fully matches linear is the ability to scroll through and choose all of your content in one UI regardless of the service.
I access my content through Firesticks or my Apple devices. It is relatively easy for me to click on an icon to move to different apps, but it still takes some time and knowledge of what is on that app. Firestick will show some of the content, especially if it's a show I've watched on the home page, but it's not comprehensive. I can search, but it's clunky and not easy to browse.
If I could just see all content in the Firestick UI and just click on that specific content, that would end the argument that "X event will die on [add any streaming service]." For example, if I can sort by "sports" and it would show all sports current showing on all the apps that I've downloaded, I can just click on F1 on Apple TV just like if I was scrolling through YouTubeTV channels.