Sure but why not do that when there's an actual new field to add? Does anyone have a proposal for a feature that needs a new version field at the moment? There's no point changing the protocol now unless there's actually a new field to add.
Anyway I still don't see why anyone cares about this issue. The Bitcoin protocol does not and never has required that all messages have a fixed number of fields per version. Any parser written on the assumption it did was just buggy. Look at how tx messages are relayed for the most obvious example of that pattern in action - it's actually the raw byte stream that's stored and relayed to ensure that fields added in new versions aren't dropped during round-tripping. Old versions are supposed to preserve fields from the future.