"no one is able to work productively for 8 hours, that's a fact" - this is not a fact at all, where is the proof? Show me the research. I would still challenge it by showing you days when I've easily pulled 12 - 14 hours for 5 and sometimes 6 days of solid work in a week with no burn out. It's all about managing energy and having your internal motivations aligned with the goals and tasks at hand. Not that everyone should be working 12-14 hours but often we need to.
I think you're definitely right that under the right motivation and for certain bursts of time you can work for 12-14 hours of time, but there has been a good amount of research and studies showing long term damage from overworking and undersleeping:
