Maybe for macOS users, because it is nowhere to be seen on Windows.
Which has always been the case. Adobe back when I checked, made something like half of its profits of Suite on the Mac.
John Nack (of Abobe) had commented on 2009:
"Last time I checked, Creative Suite sales were roughly 50/50 Mac/Win." [1]
So, the fact that Windows has more market share is irrelevant regarding whether Adobe should worry.
More relevant things would be:
1) designer share of macOS/Windows might have skewed since 2009 (but even at worst case I'd expect a 30-40% to still be on Macs).
2) Sketch alone just tackles Illustrator duties, not several other things the Suite does. So it wouldn't be enough for Adobe to lose Suite buyers over -- just people only using Illustrator.
But combined with newcomers like Affinity and co, it might.
Obviously, whether Adobe has to worry from a competitor that's OS X only depends on the percentage of OS X vs Windows users buying Adobe's program.
If N% of Adobe's customers use OS X, then Adobe can still potentially lose up to N% of its users (the OS X using ones) to a competitor -- and whether there are tons of designers "living in countries where Apple hardware is out of reach" wont change this.
(Besides in many such countries most designers will just pirate Adobe stuff too, so they're neither Adobe nor Apple's customers to begin with).
What I think is their share of creative might have dropped slightly, as some have moved to Windows (e.g. Premiere using editors, and others finding Apple's Pro machines slow release pace and high prices for customizations to disk/memory bad).
But I think they can have the cake and eat it too if they use Qt. But the price of Qt library is quite steep (USD 5500 per year). I know, I know, you can use it freely if you use dynamic linking. But still....