I politely disagree. If you think “hours spent at desk” correlates with performance, I don’t really know what to tell you.

I do work late, and over my career have absolutely pulled all nighters and weekends and been on the redeye, but that’s when I stretch or flex, and it’s simply not sustainable.

Of course there are. In my 20 years of experience I've never worked in a place which would expect people to work overtime. (this wasn't in my selection criteria BTW)

There absolutely are, but they aren't startups.

I've always been somewhat flexible, especially when traveling, but I've pretty much never worked super-long days as a routine.

You've been brainwashed.

Not true at all.

Ironically, it's only junior engineers that haven't learned how to say "no" that I've seen hold this opinion.

Most of people I know work 37 hour weeks and use sone kind of flex system where hours above 37 per week are saved and spent at later time. The only person keeping track of your time is yourself.

Isn't socialism great?

I don't work for free and neither should you.