The multiplayer game (the thing available now) will remain free including all the updates and improvements to it. What will be behind a one-off-payment will be the campaign. We've had a few bad actors spread word that the entire thing is going to be behind paywall, microtransactions, pay-to-win etc but none of that is true.

Which assets? The textures, models, and animations are Creative Commons (various flavors, mostly non commercial).

If the code is open source, one can always build a client that works with the Creative Commons assets or community contributed ones.