It's not intended to "sell" anything.
If you're saying that the advantages of having a highly developed brain do not outweigh the resource costs, then I must ask you to speak for yourself.
Quite so. However this doesn't mean that no change is explainable at a high level.
No it does not, because the habitats and the biome itself have changed over time, so the niches have been altered. You're also ignoring that changes in one species will effect changes in another (it's a bit of a bland description, but this is sometimes referred to as "the arms race"), and that the niche is not actually the be-all and end-all of determining success. Let's not throw sympatry and mutualisms out of the window just yet.
The whole point of evolution is that it describes the means by which species adapt to changes. It's fairly evident that (a) habitats have changed dramatically in the past, and continue to do so, and (b) that this has resulted in a vast number of species being wiped out because their evolutionary mechanisms were outpaced by the changes.
This doesn't imply there is something wrong with evolution as a mechanism. Evolution is not a "thing"; it is simply a property of living systems.
Earth's climatic history is one of relatively violent upheaval.
No offence intended, but it's quite evident that your perception of evolutionary mechanisms is quite wrong. There is no such thing as a perfect state in a biomic community. Even abiotically-necessitated migration alone would see to that.
Not being an anthropologist, I'm not really interested in speculating on weird new theories about the origins of man. The post you responded to was simply explaining why non-advantageous features disappear over time; you seem to have interpreted it as being something it is not.