They haven't revealed how the reward system works, but yeah, this is a concern. The level 32 version shouldn't be too bad but 5 waves still has the potential to be quite a long run. People rage quit after one RTO on a Nightfall, so it'll be interesting to see how the community reacts.
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It's been said by those who played the Prison of Elders that upon completing all 5 rounds, you are rewarded with a Treasure Key, and the center floor of the Airlock leads down to a room with 3 chests. There's no footage though, as the NDA said that they can talk about how the rewards work, but no reveal any pictures or footage of the Treasure room.