For me the most interesting point in the piece wasn't about the declining sales in the sub-genre, but about the effect that was having. Given the tougher time selling YA books the consequence appears to have been (according to the article) to squeeze British authors off the bestselling lists, leaving them stacked with Americans.
You can see why this would happen. In fantasy writing (my genre) the setting is typically a secondary world. Everyone who comes to it as a reader needs to be (gently) educated in its ways and introduced to new ideas. Part of the reason fantasy settings have traditionally been so medieval Europe-based is that the readers are accustomed to it and less time has to be spent on world building. Of course, of late there has been an explosion of more diverse settings, which is all to the good, proving that the readership is prepared to put in a little more work, knowing that the reward will be worth it.
In YA the setting is often a real world one, and the larger US population, buying a larger proportion of the books sold in English, are more comfortable with familiar settings - i.e. America. Writing is hard. It's easier the more you share in common with your audience. If you have the same cultural references, can describe the same neighbourhoods, the same experiences of growing up, then your words have more power to resonate with the readers. Sure, with sufficient skill you can get powerful resonances just playing off your common humanity and emotional response - but it's easier with the tide flowing in the right direction. British writers tend to write in British settings - you write what you know. Publishing wisdom is that this will sell less well to Americans.
My fantasy books are available in 25 languages and a great many countries. My real world sci-fi books are selling very well but are available only in English. A significant part of the reason is that they are set in London and thoroughly plumbed into the culture. That's a learning curve for a reader in India or Russia. And whilst there is an appetite for fiction from other cultures, it's normally in the literary fiction genre. Folks who want some sci-fi want some sci-fi, and they're less amenable to anything else "getting in the way".
Now here's where I join the (apparently) growing trend of jumping off the YA ship, although as far as I was concerned I was never really on it. One Word Kill and the Impossible Times trilogy have been labelled as YA because they focus on young adult protagonists. I've no problem with someone calling the books YA, but the truth is that I've never written a book that I didn't also want to read. I'm over 50 ... shit ... anyway, I'm over 50 and these are books I want to read. They concern young adults but they are set in a period when I was that age - the 80s. I had imagined that main audience for this story would be people old enough to remember the 80s or at least be close enough to them to understand the references and have an opinion about how well the vibe was captured. If someone in their teens or 20s enjoys the books then that's excellent, an added bonus, but I never wrote the books thinking "now how can I entertain 'generic teenager?'".
It does seem sad though that, with 60 million Brits as potential readers, authors are being told that should they want a shot at hitting the top of the charts then they'll have to do the equivalent of when British actors adopt an American accent to get ahead. I know it must seem like a small problem to an author from an even smaller country with a different language struggling to break into the English speaking market, but there it is.
Films have been the same way forever. I grew up knowing the ins and outs of American youth culture because Hollywood made all the films. I knew about proms and baseball and what suburban America looked like, what the houses and road and schools were like. While my US counterpart neither knew or likely cared what the equivalent experience in the UK was.
Anyway, I wrote what I know, and I know 80s London. So there it is. And actually the books seem to be selling very well in America. So maybe the publishing wisdom is beginning to be undercut as part of a new willingness on the part of readers to go to new places. And whilst London isn't as big a step experientially from New York as Tokyo or Nairobi, it still a step I hope readers will enjoy taking.