Old missions don’t spawn in the middle of the world. Vex invasions are literally going to spawn in the world. Portal opens in the sky and vex come out. Thats how the activity is initiated. You can’t have the story progress past that while still having the invasions. Not to mention the strain all the content would add up to. We have another 2-3 years of d2. Continuously adding content would slow the game down significantly. Use your brain. Don’t buy it if you you don’t want it. You’ll end up buying it any way though, seeing how simple your mind is.