thanks @Mujo

Comparisons are seductive because they give us a narrative we are familiar with that can seemingly predict the future. So I say this with a significant degree of scepticism: but I am beginning to think of CPAP as akin to spectacles.

They've been around for ages (c. 1290, apparently!) are clunky and inconvenient and there have been several new technological advances that seemingly have spelt the demise of an inferior technology: contact lenses and refractive surgery. Yet here we are still seeing a rise in worldwide sales

For both markets; the global size of the pie is getting bigger every year, more people can afford it, and seemingly superior options have draw back or other issues that for whatever reason people choose not to accept (size of slice of pie doesn't seem to be getting much smaller)

As I said, its a lazy thought process, but might hold true - it'll be fun to see how things turn out.