in the basketball example height is only a proxy for the ability to play basketball.

IQ is a proxy for the ability to do well in a job, too. I was ignoring that, so in the analogy I would have to ignore that for height and ability to play basketball.

proxies have different effectiveness or usefulness

Arguing "the other proxy is so much better that we don't need the original one" is not the same as "the other proxy is better, and that's all we need to know", and is even farther from "we can measure it so we don't need a proxy at all".