I agree it's a far better system. I also understand you couldn't develop or maintain the game keeping in mind both building systems.
But you know what? There was a very simple solution to all this. On launch of CS there could have been this little paragraph from you:
"The building rework would simply not have happened without development to link it to because the two were simultaneously designed. So, unfortunately, though we provide our customers with the new building system, since we can't maintain both, and with the new development system for their provinces, we need to keep the development raising system in the dlc, becuase it is a new feature (since you couldn't raise base tax etc. with the previous system). This will be locked for the people who don't own Common Sense. We are sorry but this is a matter of dealing with the cost of both developing the new system and maintaining the previous one. Thank you for your understanding.
However, the new system has this and those features, so we feel we are not taking away anything from your gaming experience, since even without the dlc, province development does (not) add this and this, and you will not feel any major difference.
Paradox Studios".