So I get MSFS servers have gone down worldwide and that’s fun and all, but the popup in-sim literally won’t go away and keeps popping up every 3-5 seconds no matter how many times I press the X. How are you supposed to simply fly offline if the giant red popup keeps attacking your eyes while daring…

Turn off all Live and Online Options in Data and restart your flight. That would include things like Photogrammetry, Bing Maps streaming, Live Weather, Live Traffic, Multiplayer.