> It doesn't really matter about the UK's status. Nobody is saying "well I live in a hegemon, I won't bother learning another language".
That's a straw man.
> For the most part, people learn languages because they are useful.
Duh. But how useful learning a new language is to you is mediated by where you're living and it's local influence. As an American, I would have to go way out of my way in order to use a new language, let alone find it useful. This is largely due to the cultural and economic weight that America exerts on the region. Literally everyone I've done business with who isn't a native English speaker taught themselves English specifically to move to or do business with Americans. The UK is not in this position.
Also, again, size. I have to travel a very long way before I hit anywhere where English is not the overwhelming majority language. This also is a factor. If I want to learn French and use it, it's a $600 and 14hr flight for me to actually make it to Paris and use it, while a Eurostar ticket from London is $60 and 2hrs. This absolutely affects how useful a new language might be, even for pleasure, and is exactly why my attempts to learn French petered out.
> I'm happy enough with that because speaking another language well represents thousands of hours of effort that I was happy to spend elsewhere - it is a huge opportunity cost.
YMMV.