but it's not clear why being known to be muggable is a disadvantage, given that here in the real world we don't pay the mugger regardless of our philosophical views.

Being known to be muggable attracts people to give it a try. But if we don't pay the mugger in reality, then we can't be known to be muggable, because we aren't.

You are 100% sure there is no other way to get as much utility as the mugger promises at any other time in the future of the universe.

It doesn't seem unreasonable to get quasi 100% if the amount the mugger promises is sufficiently high ("all the matter in all the reachable universe dedicated to building a single AI to define the highest number possible - that's how much utility I promise you").