Once the time counters are gone, the permanent is sacrificed
Prosper likely isnt going to be the Treasure Token generating machine that it could be when built around, but this card ultimately creates Treasure Tokens and provides card advantage, which is good enough for a lot of cases
Its alternate casting costyou exile a green card rather than pay 4 manamakes it a free answer to cards like Chalice of the Void, Leyline of Sanctity, or Ensnaring Bridge
Adding these impactful cards to your mana base mitigates flood and maximizes your interaction suite
Urza can play a variety of different strategies ranging from Polymorph-based combos to extremely resilient stax
In Final Fantasy, this means Noctis, Prince of Lucis can bring back artifacts permanently, or Relentless X-ATM092 can return from the battlefield hangs around forever