My model is a bit simpler: model quality is something like the logarithm of effort you put into making the model. (Assuming you know what you are doing with your effort.)

So I don't think we are on any sigmoid curve or so. Though if you plot the performance of the best model available at any point in time against time on the x-axis, you might see a sigmoid curve, but that's a combination of the logarithm and the amount of effort people are willing to spend on making new models.

(I'm not sure about it specifically being the logarithm. Just any curve that has rapidly diminishing marginal returns that nevertheless never go to zero, ie the curve never saturates.)