I realize this is a very old thread but I'm putting my 0.02 in from my experience, it feels like it's actually a steep incline in difficulty at approximately 100k pop, and up until then the population just doesn't demand very much, I noticed my first dip at 85k, but it recovered quite quickly and I didn't even really notice it as I was busy dealing with other things, but it happened again at 110k and it hit hard, brought me all the way down to 90k ish population and my industry was flatlining as a result, it did however recover after quite some time and I just used the period in between to go through my services and reinforce them and tweak my finances a bit.
Here's a couple of points worth noting:
1: If your population begins to decline heavily don't stress about it too much, it will settle and rebound
2: If you want to continue to encourage growth it is possible, but by 120-140k pop the citizens become VERY demanding
3:Try to keep uneducated in the range of about 20%, don't give in to the demand for more schools too quickly, uneducated people are required to make light industry run and are heavily required in light commercial, and although high density commercial and offices are tempting during those "dips" in influx you will see a rather heavy die off in those zones, that and commerce needs industrial to function.
4: Don't focus too much on High Density Residential, remember to build some low density residential, personally I used those periods of outflux to build what I call "wealthy neighborhoods" which are usually attractive neighborhoods with lots of parks, fire, medical, and police coverage, as well as adequate access to education. I found that shortly after beginning to build those small "wealthy neighborhoods" I began to get my residential demand back. (If you think about it realistically, it gives people the impression that they are moving to Hollywood and not Boston, meaning they have the "hope" of striking it big one day and owning one of those mansion like homes)
5: Commercial demand is largely contingent on tourism, the more of a tourist destination you can make your city the better your commercial will do, of course losing 20% of your population in a short period of time tends to kill a lot of businesses through lack of employees, so temper the urge to build commercial a bit.
6: Don't build residential to immediately meet demand, wait for residential demand to get close to the half way mark before doing so, part of the flatline is that the high density residential structures will often be partially empty after a mass die off/exodus, so most of your new demand will be eaten up by the empty space in the already functional and not entirely abandoned buildings. It also helps having some existing residential demand as during those "die offs" the population waiting to move in will move in to replace the population that is dying off, this will dramatically reduce the negative effects of those die offs.
My only wish was that if you left the game running for long enough eventually those "die off"/"rebound" periods would level themselves out, but people just seem to continue dying in waves for eternity.
