"Well, if B3 is able to do something faster with the same level of code quality, then I'd say it was a mistake in LLVM to not make the same decisions. "

It's more "nobody has had the time to erase this particular technical debt yet". Chandler, for example, is focused on erasing the debt of the old pass manager first. The B3 guys, had they focused on doing this in llvm instead of writing a new JIT + new passes + .... probably could have done that in less time.

But at this point, there are concrete plans to erase it, so as you suggest, it is highly unlikely advantages will persist in the long term :)