First, a few tips:
- Go through every information layout possible and make sure your citizens are covered by all of the necessary services
- Check your building levels for Residental, Commercial, Industrial/Office and try and get them up as high as possible
- Traffic can make or break your simulation. I recommend using the "Traffic Report Tool" mod to help wrap your head around why a citizen is going where (it might help you spot missing road links!)
I am currently having your exact problem. My current has 131k population but zero demand:
As you can see my RCI is almost zero across the board. Something else must be going on here. My traffic has hardly any red spots, tons of services, and high land value in the right places. What's going on? It turns out high unemployment can cause your city to be undesirable to move in to:
But how do I have unemployment if my industry demand is zero? Personally, I have an un-even location placement of homes vs jobs.
I think this is causing my high unemployment even though there is no demand for OVERALL RCI. The industries that are far away from homes are complaining no workers are available while the residents that are far from jobs are left unemployed.
I also have a few other problems like an unnaturally low commercial percentage thanks to the park bug (BTW, thanks for the mod heads-up, I'll check it out!)
As Dennis noted, service is a huge part of this - all the services in the game only work if transportation is available (of course, power uses power lines, water uses water pipes, so they aren't affected by road traffic).
This gets worse as the city grows bigger - it's very easy to have all your service trucks stuck in one traffic jam. Note that the service buildings don't have a finite range, the way it's indicated in the UI - that's just the range where they're effective. They will happily travel through the whole city, through three different traffic jams to get to a point where they are needed, and then back again. There is a limit to how long they can spend on a trip, but it's pretty long. Something that looks like a fun traffic jam in a 20k city will completely paralyze a 60k city. Seriously, I've had a city drop from 80k to 50k over a few weeks, just because of traffic jams!
Industry can only work if it has the resources. If you're seeing many abandoned buildings, this is one likely cause. This is much more important when you have e.g. wood industry without a forest - all the wood has to be imported. Industry creates huge traffic in the mid-late game.
And I've observed increased taxes after fixing traffic issues. I'm not sure about the cause, it might be that commercial zones produce more money when they get people-traffic. Or it might be the better service access, who knows.
And of course, traffic jams add a lot to noise. This can be countered (trees), but it's a factor.
Best Answer
There is a very highly rated (5*) wonderful article / guide for Preventing "Not enough workers" and minimizing Death Waves written by Blake Walsh in steam's Cities Skylines Workshop Guide.
In which it is been specified why people or moving out from the city. The short answer is following but please read the guide for more information on this.
In the guide you can find the following answer in the section starting with "But the most pertinent part of the lifecycle is as follows":