or paid updates or monthly subscription or any other form of money injection, development will need funds and spending years on game post release require some serious funding.
so paid expansions (same as most games now are doing with DLC or season pass) is the best way, unless you can suggest much better financial solution.
those expansions will really be game changers, they will not just add some silly new assets to play with, but cardinally change our game, FPS, planetary landing, thargoids, all that will bring loads of new unique missions, game play mechanics, assets (like fps weapons, ground vehicles, settlements on planets, wild life hunting etc. etc. etc. etc. etc. etc.), so if paying for some assets and maps in COD is ok why here we should take all for free and as granted?
anyway there are planned 3 expansions and probably they come out once every 1-2 years, so even if they cost 35 pounds they will cost you 1-2 pound per month saving, is that an amount that you consider too big for completely new experience?