Complexity already builds with each era. Raw materials you were exporting in previous era's can be made into more complex exports later on for higher profits as the raw materials don't supply as much profit and so on. The size limits of the islands make it more profitable to focus on a few economic chains that lead into more complex industry in the next era rather than just random everything.

Your population does have demands, some random and some depending on which story mission you are playing.

And your internal economy feeds directly into your populations happiness, food variety and luxury goods produced go into the grocery stores and shopping malls, you can see what they supply when you produce them. The more variety, the happier your people and so on.

If you're just plopping down a few randoms and "that's it" it sounds like your just playing sandbox and not actually playing the games missions or understanding how deep the economy can go.