Steam does not respect user privacy, at all:
In Steam's privacy policy[1], Steam details that it collects the following user information:
Name
Address
Credit Card Number(s)
e-mail
Age
IP Address
Device Unique ID
Chat logs
Forum posts
Voice Chat Recordings
Hardware Enumeration
Steam also confirms that it shares this information with third parties. The implications of this are as follows: Steam knows your name, age, where you live, your banking information, and what your e-mail is. Steam shares this information with other companies (at least, to the extent allowed by law). Steam can use your IP Address to track where you are to the nearest county and can use your Device Unique ID provided by the fingerprinting spyware features inside Steam to track your usage habits across devices that you use. Steam also records all of your communications with others through its social networking and instant messaging services, such as all chat logs, voice conversations, and forum posts, and can share all of this information with third parties as well.
Steam is a self-updating proprietary DRM launcher. It serves 4 main purposes: act as a webview into the Steam store (in which Valve takes the lump sum of each and every transaction made), download and manage your games library through their Steam content delivery network and library UI, funnel users into its social media platform called "Steam Community" and most importantly enforce software digital restrictions via Steam DRM.
Though developers can disable steam's DRM check and users can take any number of obtuse workarounds to crack steam security: the intention is to make it so that Steam becomes the singular monopoly and model for video game distribution on PCs.
Valve has also been guilty of inducing underage gambling addictions in young children (as young as 12) and has profited immensely from pioneering the virtual "skins" economy where users spend real world money to obtain artificially scarce digital aesthetics. If your "virtual economy" has a market cap in the hundreds of thousands of real dollars then it's safe to say that you've built an extremely toxic space that preys on people's psychology. Search "CSGO and gambling" on any site and you'll find dozens of sources and documentaries on exactly how little Valve cares when it's their bottom line going up. Valve's Steam social platform also perpetuates, this, through frequent sales and advertising that's impossible to turn off. Through "Steam trading cards" that fire up the dopamine in your brain to keep buying, to keep trading, to wonder if you can fork just a little bit more to get a new game or skin.
Valve supporting "Linux" is not a plus in Valve's part but the culmination of years of thankless work by thousands of volunteers across dozens of projects to actually create a space that Valve could contribute to. In reality, Valve isn't doing anything different compared to most large tech corporations when it comes to publishing new work and making contributions.
Steam is also intentionally a terrible product on a technical level, instead of publishing a Steam API SDK for app developers: Valve has realized that creating their own google chrome-in-a-box launcher would allow them to double their launcher as an uncontested advertisement platform for all their future products. This slow, memory-intensive archaic 32-bit application for Windows and Ubuntu stays always on in the background, ready to manipulate your psychology into playing more video games and interacting with the platform as part of your daily computing use (not too dissimilar to Discord).
Steam also is, like most US-based social platforms, a vector for alt-right reactionary content to spread. The content moderation guidelines in Steam are guided by how far Steam knows it can take it and again, are not so different to any other tech corporation.
I'm sure some 
will come and defend their treats and how steam is the lesser evil and all that and how it's bullying to come after their treats, but I don't care, you're perpetuating harm by using Steam. You've given up before you've started and have become dependent on a multinational corporation who can and will get away with it again and again.
I've had a steam account before, it had hundreds of games that I've accumulated through years of Steam summer sales (probably hundreds upon hundreds of dollars in value in games alone). I decided to delete my steam account because I wasn't comfortable having my entertainment outlet be under the whim of a company who represents everything I hate about the tech industry.
I started to read/listen to theory, I take walks, started a fitness routine, I enveloped myself in indie webcomics. I pirate all my games and have a retro-game library at my fingertips. Life got a lot simpler once the "[[ 90% ]]" tag was being blasted in my face when I opened the Steam app, manipulating me into buying slop I won't even play or enjoy
We need to start supporting game developers directly, cut out the parasitic middleman, because sure as shit you're not supporting your favorite games when they take an enormous 30% revenue cut just for their monopoly.
True that last part was lib. The only real solution is to either break up valve in a anti-trust bid or to just focus on building a community of libre game dev co-ops who are supported by the collective rather than given a small portion of the profits.
Neither solution is particularly possible under capitalism.
Exactly.
Also one of the worst part that you understandably waved off, is that it became and stayed an oligopoly only because their (real) competition has only been interested in offering something worse, so it is enticing to people to use Steam and think of it as a well intended company when in fact it just has different priorities. To most people it appears as the better end of the stick (« the lesser evil » as you called it), but that mean they won't move on regardless of how bad it is, they might not even believe (or choose to ignore) something better could exist. In a way it's a bit worse than just being dependent or brand tribalism. I believe it's still interesting to point out because not many companies are like this.
Tbh I think its also just purposefully (on valves part) impossible to compete with valve on its scale. The network effect of being the largest games platform means that either you go the epic route of selling at a loss or you become another miniature storefront for your own first party titles and also sell on steam.
If valve was compelled to release a steam SDK that could be used to build alternative launchers then that position would be more fragile since it would actually create unified launchers.
Oh I'd say it's impossible to compete regardless of their intent, that being said yes they are absolutely taking advantage of this.
I don't see how having unified launchers would change much though, I mean I like the idea for sure, but it's not the software that made Steam indistinguishable from a monopoly, among many things it's their store and how both studios/developers and gamers are compelled if not forced to use it. Even going through different stores won't change much of that because most studios/devs still have to publish there.