Its not like you will have to buy everything(because yes, new stuff will come and will keep coming) to be competitive(best commanders are free, except lightning war and elite troops).
Its also not EA battlefield or Apple anything, so you won't feel as inferior human being if you don't hoard all of whats in shop.
As someone whose budget is pretty tight the commanders (and the collector's edition upgrade for that matter) were rather upsetting when I had enough money to get the base game. Yes, you don't need that extra content but I do enjoy the game and like to use most of what's available for it.
I did end up getting a lot of the commanders over time and I don't mind supporting Relic when I can, I'm just bitter of how the industry is regarding "charge now, charge again later" mentality in general. I guess I just miss the older days.