As always, you can't make rules out of exceptions. I could apply the exact same reasoning to any extension, highlight just a few sales to claim the TLD is being 'successful'. Flukes don't make a market.
Seriously, there is a lot more value and
credibility in .uk than in .xyz. Anybody who is in doubt about that needs to drop out of domaining now

In my view, .xyz will never be a credible extension. The roisterous marketing doesn't change the fact. I think it's mostly a fad, and it's going to lose steam at some point. Previously-released extensions like .mobi, .co, .asia, .me etc have recorded large sales too, but look where they are now...
The value of TLDs is not driven by sales, but by development. For that you need a critical mass of end users willing to put up quality websites. By that measure I don't think that new extensions are doing too well.