Does "Demand" ever go away?

Like for instance you have a decent city going and all your basics up and you've filled the map.

Does the Residential/Commercial/Industrial demands ever "even out" and stop going up? or will they always eventually max out assuming your city is "self sustaining".

No. The secret is that buildings that employ sims have a range of how many workers they can hire. Employers have a minimum number workers they need for the building to function, and a maximum number they can hire.

Even if a building has the minimum number of workers to be functional, it will still demand the maximum number of workers.

And of course when you add residential to get more workers, you also get more shoppers. The shoppers demand commercial, and commercial demands industrial (for freight).