I have just hit this limit.
200 hours played. I have played only one map, the map I started when I bought skylines. 419k population. I do not wish to start all over, I was enjoying my map, my city. Why start over in the first place if I am just going to hit this limit again?
Hopefully you had autosave enabled or manually saved recently.
You can easily get it much bigger, just install the CSL Stats mod and Show Limits mod to monitor all of the limits. But you'll need a previous good save first.