I'm not suggesting there is anything magical about 60 minutes or that power suddenly falls off a cliff at 61 minutes. So if you want to say that FTP is sustainable for "about an hour", I could agree with that. But using your example of 70 minutes vs 50 minutes, I find it hard to believe there are people out there who can't maintain a higher average power at 50 minutes versus what they can average for 70 minute. It might not be a big difference, but there will be a difference.
Anyways sorry for taking the thread off-topic.
I think your description is consistent with 'pretty flat'. There is a generally accepted model from Monod and Scherrer which indicates that maximum power for a given duration is P = CP + AWC/t where CP is critical power or the power one can maintain indefinitely and AWC is anaerobic work capacity. AWC is essentially the 'extra' energy you can dole out quickly during a sprint or more slower over the course of an interval.
For example, a TT specialist might have an AWC of 15kJ and a CP of 300W. For a 20min interval the maximum power would be 300 + 15000/(20*60) = 312.5W. Similarly the power for a 50 and 70 min interval would be 305 and 303.6 respectively or 'pretty flat'. If a rider doesn't ride with uniform power the curve can be even flatter.