Delaying the move to Windows 7 cost them vast amounts of money due to more frequent crashes and reboots, less efficient operations, greater susceptibility to malware, and generally higher support costs etc.
It didn't save them any money because they were almost always paying for Windows 7 even while they were using XP. (Either they paid for Windows 7 and used the downgrade option or paid for it via Software Assurance.)
They didn't save the cost of moving to Windows 7, they just deferred it to a later date when the move was more expensive.
Microsoft tried to help them to move by giving them a free copy of Windows XP in the form of Windows 7 XP Mode. That wasn't available in Windows 8/8.1/10.
So, as far as I can see, sticking with XP was a really dumb move. I'm sure there would be special cases connected with software that couldn't be upgraded and/or expensive peripherals, but still, in the bast majority of cases, they could have done a Windows 7 upgrade leaving just a few instances of XP.