FFMPEG alone (well, with the pulseaudio loopback) can do this. It can also stream to stuff like twitch with no extra software. OBS is great but I feel like it's overkill for a lot of what people do.
However this give me an idea about a tool that would export scene composition and video output settings to ffmpeg argument for easy reuse. It is true that if you don't have an usecase where you switch scenes it could be handy to start a record/stream from a command line. For example I have been ripping some videos from my streaming services so that my daughters can watch the movies on a plane, works great for that. Didn't bothered using ffmpeg because the preview allowed me to be sure I was recording the correct window without realizing 2h later I had recorded trash. Surely there are better ripping tool but at least this way probably do not attract attention from said streaming services as you are recording in real time.
I love ffmpeg as much as the next nerd that tinkers with video but.. it's video. It's so much nicer to interact with it using a GUI.
Not to mention if you did something wrong you could just change it in OBS rather than kill the stream and tinker for half an hour