The fact that the slope of the growth line is not 1 does not imply that economic expansion is not tightly bound to population expansion, or that economic expansion could continue under population contraction. Consider the simplest possible linear model Y = beta * X + noise where beta != 1; to say nothing of nonlinear models or interaction effects between population growth and other factors. Surely output per capita matters, and the true data generating process is far more complex, but you have not come close to demonstrating population growth and economic growth are decoupled through this example.