The Champion's Survivor feature grants at least a ~77% chance to come back on one of your death saving throws

I was looking at the Champions 18th level feature, Survivor, and more specifically the "Defy Death" benefit:

Defy Death. You have Advantage on death saving throws. Moreover, when you roll 18–20 on a death save, you gain the benefit of rolling a 20 on a death save.

In practice, the Advantage plus the higher range of success make it very likely that you'll gain the benefit of rolling a 20:

85% chance to NOT roll a "Critical Death Save" - (Rolling 1-17 is an 85% chance)

Advantage means you're taking the higher of two rolls - (85% * 85% = 72% chance)

Don't forget Champion's Heroic Warrior feature, which grants Heroic Advantage every turn, which means one more reroll - (72% * 85% = 61% chance)

Assuming you don't roll any 1s (which is incredibly unlikely given each roll is rerolled twice), you should be getting at least three death saving throws - (61% * 61% * 61% = 23% chance to NOT roll a "Critical Death Save")

With a 23% chance to not come back after 3 saving throws, this means that there is a (100 - 23) 77% chance to gain the benefit of rolling a 20 on a death save after three turns of death saves.

Ultimately I don't think this is overpowered or anything given it's an 18th level feature, but I think it's really cool that Champions basically can get themselves back up in 3 out of 4 cases.