if they went with the old expansion model, you would have to pay for it... so what you seem to be commenting on is you want all new stuff for free... you can still play it the old way, just like you would be able to play it the old way if they used an expansion model...
No he doesn't want the free stuff either. He doesn't want to be forced with part of the free stuff and then needing to buy an expansion to make it work like intended.
Old skool expansions had the original content and then changed lot's of game mechanics, adding factions and things. But you did get the whole package in the expansion or nothing. For this DLC specific that would mean the old building system as it was for everyone that doesn't buy the game, the new building system with development for everybody that buys the DLC.
Now I don't think it's done from a money standpoint only, I think the reason is more that the building systems are so differently that it would be a problem for future patches and DLC's if part of the players kept the old system. But it also was such a change and overhaul that I can think it took lot's of manhours to do and so they couldn't make the entire thing free either, because then it wouldn't pay itself back...