Why are the buildings and zones completely disappearing in Cities Skylines

First, a few tips:

1. Go through every information layout possible and make sure your citizens are covered by all of the necessary services
2. Check your building levels for Residental, Commercial, Industrial/Office and try and get them up as high as possible
3. 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!)

I haven't found any (official) evidence to prove my hypothesis following now, so be aware that it might be wrong.

You say that you have no industrial zones at all. So, let's check this chart first:

You see that there's a production chain which requires specialized and generic industry to create goods for your commercial buildings. If you had that, you could easily lower the import rate. However, you don't have those buildings or zones so basically you import everything. You can read here too that offices are a clean alternative to industry, nevertheless they don't produce any goods.

• First off, there could be an unknown import cap. Really unlikely, but possible.
• As stated here, import trucks, ships or trains don't use the nearest entrance to your city. They spawn wherever they want to. So some trucks could be already on their way, however, they're taking ages since they have to drive through your whole city. That, too, would explain why some ships just have cargo of 50%. Maybe there are still boats on the sea coming from the bottom end of your map to your upper end. In the meanwhile, other ships spawned recognized that they don't need 100% cargo because other ships already have the cargo.
• Also, do your commercial buildings import at all? Stated here again, some buildings not connected to any industrial area nearby do most likely not import stuff.

I guess the whole problem is that you miss industrial areas. Your problem isn't well documented nor reported because it doesn't occur that often. Most cities have industrial areas, you and few others don't have them and are suddenly facing the problems of too less goods. As you can also see here, offices have some downsides:

• Fewer jobs.
• No goods.
• Less commercial tax income.

As a fellow city builder, I can just recommend you to build up some industry. The flow will be way better and less troublesome. Yes, you have pollution. Just be a dirty (real life) mayor and hide anything negative from your citizens by placing it far away from them and covering it up.

As said in the beginning, I don't have any official evidence supporting my hypothesis, but I hope I could still help you with it.