When a wrestler becomes a wwe superstar he or she must sign a contract, and with in the contract it will tell you how long the contract will be valid.
When the contract comes to an end the wwe board will then decide if they want to keep your contract going or to let you go depends how much money you can make the company.
If the wwe board thinks you will make them money they will renew your contract, Your first contract is valid for 10 yrs the next one is for 5 yrs.
What people don't know is with in the contract there must be some sort of attorney read it and sign it. You can have your own personal attorney or the wwe attorney. The reason for this is because if you stay with the wwe until retirement or forced into retirement like a few wwe superstars have.
The contract plainly states if you for any reason retire or forced into retirement the wwe will have to pay your remaining years left on your contract .. Daniel Bryan was forced into retirement due to an injury, instead of paying his remaining years on his contract they gave him a job as general manager on smackdown live.
Edge as another example, Edge had to retire due to a neck injury, instead of paying his remaining years on his contract they gave him a show on the wwe network. Of course he had a short run with a tv serious, But still WWE did have to pay his remaining years on his contract ... Thats how the WWE contract works ...