Windows XP was still running on 10.9% of all desktops in March 2016, per www.netmarketshare.com. 7.4% as of April 2016 per http://gs.statcounter.com/#desktop-os-ww-monthly-201504-201604, but either way, with 1G desktops out there, per Microsoft, that's tens of millions of XP seats.
To put that in perspective, per Net Applications’ figures, as of 8 months ago Windows XP was still the third-most popular desktop OS, trailing only Windows 7 (51.9%) and Windows 10 (14.2%).
At that time there were more PCs running XP than Windows 8.1 (9.6%), and all versions of Mac OS X combined (7.8%).
My own machine-of-all-work runs XP, too. I do own and have installed a copy of W7 on another machine, but don't use it except when I positively must have more memory than XP allows and must for some reason use Windows rather than FreeBSD. To re-use the comment Sir Tony Hoare made (about Algol 60), XP was a great improvement on its successors.
What are the chances of getting a version of Designer that will run under XPsp3? Or FreeBSD?