You don't understand the definition of rightful.
They have no rights to any land on Azeroth, especially not land that has been inhabited by actual denizens native to Azeroth for thousands of years. The only land they have rights to is on Draenor.
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Well, maybe they shouldn't have blown up their planet by opening portals to invade other worlds then... Maybe they should have been happy with the one they had and not made a pact with the Legion and drank the obviously-a-bad-idea demon blood. They have no one to blame for that but themselves.
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Trolls would have never come to be without the Titans defeating the Black Empire, and then seeding life on Azeroth, which then had it's evolution accelerated by the Well of Eternity. They evolved from that seeded life, they did not come about naturally.
All life on Azeroth, be they curse of flesh humans/dwarves/gnomes, or trolls, tauren, and elves, all owe their existence to the Titans...
And technically the Titanforged races' ancestors were on Azeroth before the trolls were... So... An argument can be made that they have a stronger claim than the trolls do.