I don't understand how you can call any of them nor the coming Muslim DLC even "semi-necessary". You have the "full" experience already. New features will be added to it for free with (seemingly) every new downloadable expansion, so you'll benefit from it even if you, to example, doesn't want to play as Muslims. In turn, it is the continual income from the DLC's that will make sure they can keep patching the game until it is as bugfree as possible (and if you know Paradox history you know they have had issues with having to abandoning the patching efforts of "minor" games before because it just wasn't economically feasible to continue on them - EU:Rome for example, was left hanging with game crippling bugs (that was caused by the latest patch no less) and is afaik only worked on in private by a single dev). Not only that, one of the other reason they switched to this new - "modular" as they call it - system is that it won't force you to buy expansion #1 and #2 in case you want expansion #3 and you will still receive patches for the original content/old expansions without having to buy the latest expansion, which was the case before. So in this aspect Paradox is actually forcing you to buy expansions/DLC's less now than they did before.
As for their company size... I think they're too big to be called Indie any more as well (though I'm also uncertain whether Paradox the Publisher is a separate company or not), but as I understand it they still hardly break even most of the time - they have a niche audience and a shaky reputation among non-fans, neither which makes for great sales. CK's unordinary long developing time was possible because the success of Mount&Blade, I believe (and/or possibly Magicka? Can't remember when that game was released), which was developed by a Turkish Indie studio and Paradox only published. So at least to me it seems like they would have trouble paying for the time they need in development from their own treasury alone.