In this case, the 2 things most likely to force an upgrade - and the expensive re-validation - would be the supplier of the control software ceasing support for the version of their product running on Windows XP, and failure of the existing PC hardware, as it is unlikely that XP drivers for new replacement hardware will be available.
I'm sure that the "powers that be" will have considered and mitigated the risks noted above, and presumably in the company's annual budget there's an amount set aside every year for the eventual and inevitable upgrade - just like an engine fund.
But given the isolation, there's no reason that XP can't go chuntering along happily for as long as the hardware and software holds out.
SD