You do have control. You installed Steam. It is just evil because it "just is" isn't an argument. The resources are irrelevant and it does nothing sinister to your PC.
The "resources are irrelevant" argument is entirely case dependent. The word evil can stay out of the argument too.
You're technically right that you have control over whether to install steam or not, but the point is you don't have a choice if you want to play some games. And it's fairly reasonable to say that the requirement of steam in many games is nearly arbitrary other than for the DRM function. That is to say it's not really essential to the functioning of the game, like say, the operating system, or DirectX, or an internet connection, would be.
As I said in my post, steam uses only a tiny amount of resources, which makes the argument rather silly for most people to make because they have computers where a tiny amount of resources won't make much difference.
And demonstrating your expertise with computers doesn't make the argument against you any less valid. Several months back I went into quite a bit of detail about how I was having trouble getting steam to run in offline mode when my internet connection was temporarily dodgy thanks to my ISP and the local network infrastructure. Problems, however 'minor' or 'technical' they may be, do occur due to having steam as part of the process. These are extra to the normal technical problems of a PC game.
I used to run a script which turned off all non-essential services so I could maximize resources for gaming. However now on modern equipment it just isn't relevant technically.
I can guarantee there is really sinister software on most every PC out there unless you are running a web browser with scripts turned off. Web browsers and other common programs have all sorts of vulnerabilities and spyware.
People are entitled to the opinion that they just don't want it. However most of the arguments against it are pretty much silly. A few people have technical problems with it, which is annoying, but that kind of thing happens with every PC game.
Of course a lot of the arguments against steam look pretty much silly. That doesn't make all arguments against steam pretty much silly, does it?
By the way, the "there's already sinister software on your PC" is a slippery slope of an argument. This is because the existence of such software on your PC shouldn't justify adding more. Not that I'm saying steam is sinister, of course.