Do you think it's likely that the flat demand trend will continue or even decline? That graph also shows a (shorter) flat demand area in the late 70s early 80s.
no. with the rise of electric cars, energy usage will shift from electricity and oil to all electricity. total energy use per capita won't grow too much, but demand for current will rise. also, stuff like heating oil or gas appliances may be replaced with electric too.
My expectation is for it to mostly stay flat and not decline. Things like rooftop solar will erode some demand, but population growth and electric vehicles will add some back. I think the more urgent change will be eroding demand for large generating stations like coal and nuclear, and to some extent gas, as their energy is replaced with wind and solar. Each generating technology has different benefits and impacts, so managing these changes will take deliberate effort. And not everyone will be happy with how it has to be managed.