>> so you have to spend another $250 to get the newer version
But why? I still use Office 2016 and I haven't yet come across a document it wouldn't open. What am I missing out on?
Not sure about Word or other apps, but in Excel there are many useful functions added in the later versions like XLOOKUP, LAMBDA, LET, FILTER, MAP, REDUCE, GROUPBY.
Because you no longer get any security updates.
Some security updates, some features, and if you're unlucky and run MacOS - the ability to save files.
It's less new features and more security problems. There are patches for office 2016 though, and your use case might have a low security risk if you're not opening that many files people send to you.