We won't have enough money for everything, and even if we buy it, the information might be incomplete.
You wrote that Rel'sa should defeat Burgurt in 4 turns.
I don't know if you considered the scenario where Rel'sa damages Burgurt, and then the latter hits and heals a billion health, but let's assume you did.
Out of these four throws, just two unsuccessful ones are enough for us to lose everything.
How statistically likely is this? In my opinion, it's quite enough.
Added 3 minutes ago
I also want to note that if Rel'hiras fails his ultimate and doesn't get kicked in the butt (however, with that armor for 8 coins, kicks are not scary for him), then he can definitely run around the arena away from Butarg. Fortunately, there are abilities that allow you to attack even like that. And then the cooldown of the cleave passes, and you can try again.