Hopefully modders would reverse engineer the executables to talk to their own servers (presumably it's not as simple as modifying a DNS server to return a homebrew IP address, and there are baked-in SSL certificates) and get the maps that way...

Hah, imagine doing that and streaming a map of Mars. Or the heights and structures from a Minecraft map. Or a Sim City map, Sim Copter 2020 anyone?

Afaik, there's an offline mode. It seems like you're forced to complete an online update before you're allowed to start the game proper though and you might be stuck with the original data on the discs even if you could skip that, which would sort of suck.

This is correct.

The game client is about a 91 gigabyte download which will include all the aircraft and a low-resolution copy of the entire world. This offline data is fully functional.

You only need to be online for multiplayer, live air traffic data, and to stream high-quality versions of the world.

Also, the game has an option to manually add areas of the world to the high quality data cache, so if you know you're going to be without Internet for a while, but still want to be able to fly over certain areas with high-quality imagery, you can do that.

Isn't there an online check to ensure you have a license?

I would hope that if the game servers were being taken offline permanently, then the license check would be removed.

Even if Microsoft chose not to remove the license check, it would certainly get cracked. Cracks likely exist already.