plenty of other games with similar structure as destiny keeps all the content there for vanilla players to play in dailies or what have youIts an always online game thats built around grouping to do content. If you don't keep everything together for the people who are upgrading (which is the vast majority of players) it fragments the base.
Basically if you aren't going to buy every expansion and keep up with the game its best to play something else. The game was not designed for you. People need to stop looking at Destiny like something like Halo. Its entire structure and game philosophy is different
i'm going to use ffxiv as an example, which just got an expansion 3 months ago
all the daily roulettes from a realm reborn is there for people to play still, despite whether or not they've upgraded to heavensward. they added new matchmaking specifically for the heavensward content (or incorporated the heavensward content into the existing leveling roulette, but the dungeons that are randomly selected in the leveling roulette are added as you unlock them instead of preventing you from running the roulette until you unlock all the dungeons)
instead of having daily nightfall or daily pvp or whatever being randomly selected from content you as a player have access to it's randomly selected from all available content in the game and anyone who doesn't have that content is unable to do it, even though the icon is there staring everyone in the face (while simultaneously yelling at them "taken king required")