The first option is exactly what they wanted to avoid with the new system if they do that then there simply is no new system.
You are correct:
* Modular expansions!
This is the big one. In the past, we've released expansions that saw a loss of support for the base game, making for an "all or nothing" approach for the consumer, but supporting multiple executables just wasn't feasible for our small team and we needed to sell expansions to continue supporting the product. Hardly ideal for anyone who didn't want some of the features in an expansion. So, this is something we've thought a lot about in the last year, and have finally decided on a solution.
We've developed a new system to handle modular expansions so that you can buy the small packages that you want when they're released, and it will all be using the same executable, so whichever expansions you own your game will see continued support. There will be no problems with this in multiplayer, and if you have bought one expansion and your friend has bought another, you can still play together. You will have access to the features you bought and your friend the features he bought. Alternatively, if you'd prefer you'll be able to toggle any expansions "off" before the game starts.
But I'm not a Paradox employee, so I don't make the decisions, only offer my opinion. ^_^
(Usual disclaimer stuff, etc etc)