This comparison seems a bit off. I mean, should Rolls Royce and Aston Martin be worried that their market share is small compared to BMW and Mercedes because they sold a lot of 5 Series and CLKs (who in turn should worry about Ford selling a lot of F-150s and Fiestas)?
I know that smartwatches (and apple in particular) are trying to position themselves as luxury purchases, but they're not the same market as Swiss watches.
People who want an analogue timepiece with a mechanical movement are always going to be out there. They might even also buy a smartwatch!
In general I dislike car comparisons - but this would be much more like Rolls Royce and Aston Martin being worried about losing market share while Tesla gains market share. Sure, they aren't directly competing - but there are enough people who like the technology that they switch to Tesla as their daily driver.