I agree with you that we should be fairly compensated, but I also agree that the dogma of "40 is the max a worker can be productive, possibly less" is wrong in a large number of cases.
But its quite important to make a distinction between manufacturing work, repetitive physical labor, and mental work. Different organs are involved and they function differently. As nice as some might imagine it would be to only employ machines, in reality we still have to put bathrooms in the workplace, give people a lunch break, and admit that they're sacks of meat. They've got limits, and ignoring them will never be productive.
However my point was that when negotiating salary you can't really say "I can be productive 70 hours per week" and therefor ask for 40% higher salary. It doesn't work that way and what you negotiate is based on your skills mostly in comparison to other people who are assumed to put in cca 40 hours per week.
So even if you can be productive for 70 hours per week, I wouldn't do it and instead invest that time in your family and yourself, making your life outside of work richer and more fulfilling. Otherwise you will just be working for free the extra 40% of the time and never be compensated for it by most companies.
Finally, you only have one life so spending most of your life in the office or commuting in and out of office is quite a waste of your time. You should use your free time to enjoy your life imho and experience more things.
Is that constant heads down, in the zone coding, or does that include some coding, some management, some marketing, etc? Also, how long can you do that for? 5 years straight?
I typically work in bursts. If I don't have an outlet in between bursts (surfing, 30 minute tv show, walk, etc), the longer it takes to get back into a burst. Being in an office doesn't offer much as far as outlets go.
Obviously it isn't constant 8 hours of heads down, in the zone coding, there are things in between and also meetings and planning often takes time. I used to work longer before but as you say, that is not sustainable for a long run, you can do it for couple of years before you burn out.
I also used to do quite a lot of extra work in the evenings and weekends when I had sudden inspiration / urge to refactor or improve some piece of code. But I don't do that anymore and instead use that free time for myself and my hobbies :)
If I feel an urge to write some code on Sunday, I will work on one of my open source projects or contribute to some other projects out there if they interest me or I will experiment with new tech to keep my skills up to date.
And making your colleagues look bad if you aren't letting EVERYONE know that you are working more hours!
And the argument with extra hours working on personal projects is ridiculous. As a matter of fact I am a heavy open source contributor and work on numerous projects in my personal time, some of them already have many hundreds of hours invested time in them. I choose to work on them for free because they interest me.
If you want me to spend that extra time working on the company instead then don't expect me to do it for free. You are basically asking to own my free time which is ridiculous.
Overtimes and their compensation should be clearly defined in the contract, otherwise you can't expect to take employees' free unpaid time outside of work hours.