Worry not, you can still get achievements in older versions.
Even still, it isn't something that I want to do nor should I have to do. EU4 is a full priced game, and I have spent plenty of money on DLC in the past. Bug fixes and balance improvements are a reasonable expectation for me to have without new optional DLC breaking functionality that has always been around if I don't buy it.
Again, I'm not quite cynical enough to believe that this is an intentional strategy, it's probably just an oversight. I can't deny that the game has been trending in this direction since AoW though (Common Sense being the most egregious example) and if it continues I'm likely to just stop buying PDS games altogether.