I don't think collecting demo info is just for collecting demo itself. They want to make sure they have a good representation across the groups they care about. I've had ones which kicked me out after just age plus gender, others went the full gamut of several demo questions.

That said, I have encountered the sorts of annoyances you describe. I've gone way too far into a survey before being kicked out. Sometimes it's "sorry, you don't match" and sometimes it's "sorry, we got enough answers and survey is closed". There was indeed one case where i answered for an hour before getting the latter answer. Wanted to throw my monitor through the window.