As someone who owns TDB and HoW but not TTK...

When TDB and HoW came out all they did was add new content to the game (locked to DLC owners) and lower the accessibility vanilla players had to some of the activities (Heroics/Nightfalls) but you could still play them despite that, just not as often as people who owned both DLCs. With the addition of TTK both vanilla players and previous expansion owners no longer can:

• Play story missions on a harder difficulties.
• Play Iron Banner/ToO fairly unless Bungie disables level advantages.
• Use Year 2 rare or legendary weapons / armor.
• Access vendor weapons/armor.
• Use most of the stuff you get from Faction Packages.
• Play control / clash / etc exclusively. Now you have to play a mix of both.
• Play Weekly Heroic or Nightfall Strikes.
• Access anything in the Vanguard Playlist other than Legacy.

What I want you to understand is, when TDB and HoW launched you didn't feel like you were behind in gear if you didn't own both DLCs, and content you paid for wasn't taken away from you. That's not the case with TTK. Now you either buy TTK and keep up with the game or you are stuck with useless year 1 exotics and legendaries or year 2 greens.