That's the root issue really. The BBC is a provider of media that's increasingly irrelevant, and even if approval of it hadn't declined precipitously over the last decade or so it'd still have a big issue with the youth mostly not being interested in that format at all. It's not the only reason for declining ratings (declining quality is a big one) but it's a bad sign for the future.
That is what I thought, but the article characterizes it more broadly: "Currently, anyone who uses a television to watch content must pay the licence fee even if they do not watch the BBC"
I think they must have meant to say live television content, otherwise the fee would already apply to streaming services or anything you can watch through the tv.
Twitch streams don't count afaik as they aren't TV-like content.