That number hasn't been subject to evolutionary selection for as long as there have been cells since evolution doesn't have any way to change it. So there's no

That number hasn't been subject to evolutionary selection for as long as there have been cells since evolution doesn't have any way to change it. So there's no reason to think it's optimal. Well, I'm sure that if you evolved life for long enough it might find a way to switch bases. But this looks far harder than the sorts of changes that take a billion years, like photosynthesis or eukaryotes. So I wouldn't expect it to happen before the Sun boils off life on Earth in another billion.