EU4 regularly sells expansion packs, all of which change the gameplay in significant ways, and which would more or less be necessary for engaging with multiplayer
I see your point but EU4 packs currently cost between 15€ (e.g. Mare Nostrum pack) and 2€ (e.g. Native Americans Unit Pack) which either add a lot of new units or actual game features to the game.
Relic on the other hand surely ain't the devil of this industry but 4€ per commander appears quite a bit too much to me, considering that they don't really add new features or many new units. Often a bunch of new commanders are released where many share traits, thus personally I find 4€ per commander overpriced and which made me hesitate till now to buy them. I don't know how many time EU devs spend into balancing their units but new CoH2 commanders have also been repeatetly accused of being overpowered - either for selling reasons or due to a lack of required balancing work and testing. None of it makes Relics policy here really much more likeable.
The main thing I suspect responsible for people farming is apparently the drop system. Relic currently sells COH2 for 30€ (add ons not included) with a drop system, that has been for 3 years, up until we (hopefully) get a better one soon, a farce. I've played this game for years on a casual basis and can count the amount of useful drops I received on 1 hand. Naturally people will feel taken for a ride that way. Which might not be the case if Relic never created this drop system and just focused much more on NOT introducing overpowered commanders. But it's also system intended to reward people for playing and thus keep the customers interested to further invest their time into the game - thus it's not 100% altruistic of Relic to give away things for free.
But in the end it's likely up to your personal affection. I play War Thunder, a MMO in which I invested possibly 150€ by now (and plan to invest more) and where single tanks sell for a price as expensive as 40€, nearly the price of an anticipated AAA title and the grind is still obnoxious with them. Is such a vehicle thus worth 40€ not to say a system likeable that requires you to buy multiple of them? Certainly not. But I still do it (well at sales at least) and I don't feel any bad for it despite knowing that I pay for a totally overpriced good. Thus I can absolutely empathize with everyone who criticizes such a system in our forums for being overpriced to the max.
If you want to find a reason to justify your spendings you'll find one. I noticed this in a Warhammer 40k tabletop forum where plastic sells for the price of gold in weight, people have invested thousands of € into their hobby and react very aggressive when you adress the topic if Games Workshop might actually be milking them - a company infamous for this as much as EA is in the electronic gaming industry.
Would I thus agree with a method of bypassing those payments? With W40k people have started to melt their own miniatures. This is definitely hurting business (and if everyone would do it the development of new models would certainly come to a halt) but on the other hand I think it's the overpriced business model of GW that drove those people to look for alternatives in the first place. Satisfied customers (like me in War Thunder) don't do that. And on the other hand I doubt that such people would have had the money to invest it into those miniatures in the first place so it's theoretically no loss for the company - but that's a discussion that's getting somewhat close to the question if piracy is really hurting business as much as e.g. the music industry claims.