Whether ~$10k is infeasibly expensive or a bargain depends strongly on what workloads you're running. Single threaded stuff? Sure, bad idea. Massively parallel set suites backed by way too much C++, where building it all has wound up on the dev critical path? The big machine is much cheaper than rearchitecting the build structure and porting to a non-daft language.

I'm not very enamoured with distcc style build farms (never seem to be as fast as one hopes and fall over a lot) or ccache (picks up stale components) so tend to make the single dev machine about as fast as one can manage, but getting good results out of caching or distribution would be more cash-efficient.

Yes of course it depends, which is why I used "most developers" and not "all developers". What is certainly it not is a good default option for most people, like you suggested.