It's easy to convert them to YW7, which can still be opened in yWriter 5 and yWriter 6, so there's no real benefit to sticking with an older format. At the same time, there's no real point to going through old projects and converting them to a newer format either, if you've finished editing them. yWriter 5/6/7 can open anything from yWriter 2 and later, and YW2 files are over 20 years old now. (The original YWP - yWriter1 - file was just a text file with a list of DOC files that would open in Word when you clicked them.)
From what I recall, without looking it up, there would be some very minor things to do with exporting that would be lost if saving to an older project format - e.g. tags in the custom ebook exporters, that sort of thing.
The YW7 format is also capable of keeping fields it doesn't recognise, whereas the YW6 and YW5 formats will just throw them away. (This was important for IOS/Android/MacOS as it's much harder to keep all the versions in sync and yet they all need to be able to open a project and save it out without discarding any fields.)
This makes the YW7 format future-proof. It means I don't need to create a new format for future versions of yWriter.