MTB also went up to 135mm spacing so is 5mm more than road. So not a fair comparison. You could probably get an extra cog with the 5mm. But wheel dish, heel strikes and BB spindle length (or DS crank arm) changes would have to be addressed as well I would think. As far as lack of Campy Chorus groupset availability goes, I have no idea...
Edit: Checking merlin I see a fair amount of out of stock there as well. Its difficult to buy a groupset without a rear derailleur...
https://www.merlincycles.com/campagn...017-71985.html