Everyone that didn't buy TTK: "I'm getting locked out of content that I've purchased!"
No, you're getting locked out of TTK content (which you obviously didn't buy) but can still play anything you've payed for.
That's how it works.
WoW does it, as well.
They keep the game up-to-date with the new expansion, instead of keeping everything vanilla because someone, somewhere didn't want to buy anything else.
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If I only have vanilla SWTOR, I can still access level 50 vendors, all the flashpoints up to that level, all the gear up to that level. Everything I got with my vanilla copy of the game. If I buy Rise of the Hutt Cartel, I can access all of the ROTHC content, equip all the gear, access all the content. If I have both of those and chose not to or was unable to afford Shadow of Revan, they do not suddenly lock a bunch of the ROTHC content behind SOR, requiring me to advance to a certain quest point in it in order to do things like gain Makeb reputation again. I do not suddenly find that all the gear vendors have had everything replaced with gear flagged with "Requires Shadow of Revan"