LibreOffice, scanning, dropbox: all those work identically to Windows.

Well - the last time I tried to migrate to Libre Office my macros needed tweeking and it wasn't long into the project before I hit a big brick wall and the workaround was a major one I didn't see how to implement. I predicted a major task and subtle errors so I gave up. Open Office macro language was totally different.
The lesson is they aren't fully compatible, YOU have to be. For a non-techy user pitfalls will happen when they are not looking for problems, and after they have left a trail of debris. It can happen with newer versions of (say) Word, and when I have migrated to newer versions there was always anomalies that were hard to spot. Time is needed to inspect results in case something important changed.

But sure, if you need a Windows application, use Windows.

Not need. If the user wants to carry on with as little disruption.
Of course - if they want to play at computers then why not a tablet or a phone? Or (gawd help him) a Mac**. Methinks the clue is in the OP. A person who has been using XP for 10 years is using a PC as a tool, not a journey. I could be wrong, but ...........

PS thanx DaveRo for all your knowledge on techy things. Our last exchange is a case in point. New browser, new anomalies, takes time to realise they are there. They still are, but my workaround(s) work.

**wasn't OS X the transition to a Linux/Unix type OS?