I don't disagree. I just think it's worth pointing out to people. For the record, I did take a gap year and generally recommend them. Opportunity cost is something that advisors included had me consider and I think should be considered.
Money is certainly nowhere near the be all end all (and I'm going into academia, where prospects are much more modest). However, gap years also mean delayed payment of student loans, and less liquid finances available if you are starting a family.
Financials notwithstanding, you're also working a job you have less control over (ie, residency/med school) later in life. Especially for those considering having kids, it poses a bigger strain than if they were an attending. This may not be an issue for some people - especially for one year - but for others getting an extra year with their kids as an attending rather than a resident is favorable.
I think my previous post came off as avaricious, when that was not the point. Hopefully this clears up what I was trying to put forward. Will an extra 400k be what you're looking for in old age? Probably not. It does make a world of difference if you're trying to buy a house/support a family in your 30's, though.