Roll forward 10-15 years. Former compiler writers out of work. Gcc and llvm killed the market. On the other hand we have really good competing duopoly of compilers and lots of upper language support and diverse languages. So.. win some lose some. And it's not like rust and go and Java have to use llvm and gcc. Rust does. Go doesn't always.
We have a few AIX machines and I’ve never heard anyone refer to xlC as a good compiler. It’s an aging compiler with idiosyncratic linker flags/behaviour and no support for modern standards.
It produces better code than gcc in many cases. Which is exactly what you would expect for a compiler written specifically for IBM by IBM. Lots of fine-tuning options for the Power architectures.
You say you have a few AIX machines - which one, and which OS version, and which compiler versions? I'm on Power7 and Power8, with AIX 7.1 Don't look at AIX 4/5.
As for killing the market, there are still plenty of commercial compilers around, specially when one looks beyond the PC market.
But, yeah, being born in right time and place would be wonderful.