Svarga is a reward and it is explicitly temporary. Merit accumulated through sacrifice, gift and right conduct buys residence at Indra's court with its pleasures, and residence lasts exactly as long as the merit does. When the account is spent the being falls back into the round of rebirth. Texts describe the fading of the garlands and the loss of composure as the moment approaches, which is a striking image to attach to paradise.
The point of saying so is polemical. The Vedic ritual programme aimed at heaven, and the Upanishadic and later traditions argued that aiming at heaven was aiming too low, because anything gained by action can be lost when the action is used up. Liberation is defined against Svarga: not a better place but a release from the mechanism that produces places. Several texts make the comparison directly and unfavourably.
For readers coming from traditions where heaven is the maximum, this is the single most useful thing to understand about Indian cosmology. Heaven exists, it is pleasant, it is earned, and it is a trap in exactly the way that any good outcome inside a system of rebirth is a trap. The gods are in it too, and they are subject to the same rule.
Indra is the illustration of the rule at the top of the system. He holds the position rather than owning it, previous Indras have held it before him, and he can lose it through pride or through the exhaustion of his own merit. A heaven whose ruler is on a term of office is making the point about impermanence about as plainly as it can be made.