The issue here is trying to sell a product you are not actually capable of building. Your company sold them x amount of work over y time, when they only had x/z amount of people to do that work in y time.
It's like a factory capable of making 2000 widgets / hour taking on a contract for 6000 widgets / hour.
Management and the development team need to work hand in glove to be sure they are capable of what the sales team is selling. At those jobs I had enough sway and close enough relationships to both management and sales to ensure we were covered.
If you over-sell your ability to produce, you better hope like hell you can recruit either some good permanents, or fill up the gaps with competent contractors (often a better choice) to make any deadlines you promise.
Again, I am going to state, if you are on a death march lasting five years, or even five weeks - that's not a death march - it's just business as usual for that company. Even if they hired enough people to cover that project, they would have oversold more work and left you all death marching to the next deadline.
Don't work for companies like that. It's not worth your health, or the massive loss of income you incur, because you're working 60-80 hours for a 40 hour wage.