They still buy full games though, using old as seats to make new content for an “old” game is a great way to have more income come in. Most would probably prefer to make a new game, but that takes longer as well.
So if it’s a dlc a year at $15 for 4 years, or a game every 4 years for $60… what’s the difference in the end? Other than what you think is going on inside your head? It’s the same content, same price, same everything, you just get content yearly instead of every 4 years. Bonus for everyone since they can than use that money after the first year to maybe make the other better.