Yep the person paying £200 for a Chromebook is not likely to be the same as person paying £679 + £89 for a Dex and + £ for a monitor and keyboard.Absolutely zero chance of this happening. The type of customer that DeX appeals to would likely not be purchasing a Chromebook in the first place (and I'll argue a Chromebook might actually still serve them better--see my next paragraph). The great majority of Chromebook sales are to education---can't say I expect any schools to say 'skip the Chromebooks, let's get all the kids a Galaxy S8 phone, and then spend even more money on monitors, keyboards, and mice.'
I watched the demo for DeX--does seem to be a pretty good implementation (so why exactly can't Chromebooks or Android tablets offer a similar experience if Samsung can do this?). However I still see this going absolutely nowhere. Great, I now have a more desktop friendly way to use my phone when I stop in my office--after I purchase an additional monitor, keyboard, and mouse. What exactly do I use though when I'm out doing business? Chances are those same users will still need a laptop when out mobile and this doesn't solve that problem (i'm not typing long documents, emails, putting together presentations, etc on my phone). So if they do have a laptop as well, why exactly would they use DeX in the office? There is a very niche target market for DeX.
If anything, a laptop styple shell that could accommodate my Galaxy S8 would be far more useful in today's world, though that's also been done and it didn't work then either.
Likewise person carrying a Chromebook is unlikely to replace that with a physical monitor, Dex dock, keyboard, mouse and a little power generator just to keep them going whilst portable ...
The Two are very different solutions and practicalities let alone price.