Pi is well-defined, yes, and that's not going to change. But some notation is better than others. It would be better notation if we had a symbol that meant 2pi, and not necessarily any symbol that meant pi, because the number 2pi is just usually more relevant. There's all sorts of notation we have that is perfectly well-defined, purely mathematical, not dependent on any system of units, but is not optimal for making things intuitive and easy to read, write and generally process. The gamma function is another good example.
I really fail to see why metric vs. english units is much more serious; neither metric nor english units is particularly suggestive of anything these days. Neither is more natural. The quantities being measured with them aren't going to be nice clean numbers like pi/2, they're going to be messy no matter what system of units you measure them with.
What about the gamma function is bad? Is it the offset relation to the factorial?
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