That's sadly becoming more and more common: Instead of imrpoving the old version of the game, rerelease it as "definitive edition" with all the dlc and the improvements/fixes the main game should have. I know you are hinting to Planetary Annhilation because they are the most prominent example, but by god they aren't the only ones doing this and deverlopers really think it's a smart idea to do so.
And people buy it, so I gess it works.
I don't know if you know the game series "The Incredible adventures of Van helsing". It's a Diablo-liek hack and Slash. There have been three games but the developers released also a Final Cut game that included all three games in one. But unlike to what other developers did, you get Final Cut for FREE if you have the other three titles. That's an unlimited time offer, it doesn't matter when you bought all three games, if you have them, Final Cut is on the house. And Final Cut is also othign more than a refined and fixed version of the other three games, so if they can do this, why not other developers?