I know what
@zulu354 means by this. The so-called "poor" neighborhoods are still neat and clean with nice-looking houses and buildings. In real life they would be more rundown, the vehicles wouldn't be Porches, BMWs and Mercedes in the driveways, and there should be more trash piled up. As he says in SC4 the crime rate is much higher, and I think there was a bigger chance of fire as well.
I agree that there's no reason why a formerly poor area couldn't become higher wealth due to updates and renewal including adding in more parks, and more services. The same with a wealthy area becoming poor as the economy changes, or people move out/die due to disease.
This latter point brings up something else. What about a varying economy? The economy right now is pretty steady with little or no variation. If a city is exporting goods, for example, this should also be reflective of the city's surroundings. Off map the economy could be slowing down or booming, which can decrease or increase our business revenue and affect our local economy as well.
Suppose one of our cities is in an area that's booming. The profits are raking in along with an increase in tax revenue for the city. There is an ever increased need for workers, which in turn brings in more and more immigrants, not necessarily foreign, to live and work in our own city. This can go on for months, or years in Cities: Skylines time. Then slowly the numbers start to drop down. First there are small layoffs and as the demand wanes. Then the company profits start dropping more and more and factories close, which causes people to move away and our own city growth calms down and slows to a crawl. This slowdown could also last in Cities months time and this could have a devastating effect on the local economy and city revenues, which could cause us to try various things to boost our coffers, or the time period could be brief and only a small slowdown.
The economic model could be optional of course because not everyone would want to have a fluctuating ecosystem to build in.