The other digital distribution mediums aren't dying because of Steam's popularity, they're dying because of Steam's superiority as both a reliable content distribution platform and a social media entity. Only Uplay even comes vaguely close in terms of usability and that's with only a minute selection of the titles available on Steam.
1) You can only assert a monopoly when other viable alternatives do not exist. Other viable alternatives do exist. I'm sure I'm not unique in also purchasing products from Uplay rather than Steam, as well as physical copies fro Amazon or even the supermarket. You can't allege a monopoly unless competition has been eradicated, and despite the huge surge in Steam sales over the years their market share has effectively plateaued. Maybe 80% of PC gamers use Steam, but they don't solely use Steam. Coke doesn't hold the fizzy drink monopoly if 80% of consumers drink it, but the majority of those also drink Pepsi too.
2) Valve have been extremely receptive to community incentives. I don't know if you've got any familiarity with their entirely non-linear, non-hierarchical business structure, but I'd wager that's probably the primary cause. They're almost the polar opposite of most developers. You can't really assert that they're likely to be influenced negatively by their financial strength and market share if they're already ranking highly on both and still engaging in ethical and community-centric business practices. The problem is that, whilst I'm sure you personally find your argument very compelling, it doesn't really seem to have much of a basis in reality.
3) Veiled accusations of us all having been brainwashed by Valve (yes, they're in your posts) are typically the last gasp of the deluded conspiracy theorist. Please don't debase yourself by making them.
Exactly, to use your own example in this context makes perfect sense, y'see people that buy Coke aren't only exclusive to the product, they'll drink Pepsi too, but they like Coke, they prefer Coke and although Pepsi tastes nearly as good it's not as wide-spread has coke, doesn't have that specific flavour and more importantly, there just aren't enough varieties of it for it to be worth switching to as your primary soft drink. So you stick with Coke, maybe you buy a Pepsi once or twice every week, but we both know you'll have bought at least three dozen cokes at that point and the Pepsi is only really a substitute for when you find yourself lacking the ability to find a Coke in whatever store you're in at the time.
You have, whether you did knowingly or not given Coke far more money than Pepsi while still being relatively impartial to how or what you choose to drink. Now the same could be said of people just subconsciously using Steam, I mean think of it, everything about Steam is meant to be more addictive to you than Farmville is to a forty-five-year-old divorcee. Valve have done and are continuing to do everything they possibly can so as to make you want to keep coming back to the service, even going to so far as to implement a trading card program of sorts to tap into many gamer's hidden geek.
Now whether you say that you enjoy these additions or not is entirely up to you, but regardless of whether you like them or not, you start to grow a dependency for them. Think of the ease that using Steam allows you when you want to play one of any thousands of interesting titles, it's rather mind-boggling to say the least, the system is neat, tidy, clean and tight, yet it also shows off just enough skin for something to always catch our eye. Let's say you buy a game off Steam for the first time, you've got a nice new, easy to navigate area where you can play your new game at anytime you see fit, then buy another because you think it's easy to use, and then another and another until eventually it gets to a stage where the majority of your time is spent on Steam and most of it isn't even spent gaming.
I know for a fact that at this point I've spent more time building, creating and looking for modifications for Skyrim than I actually have spent playing it, but all this while I've still been connected to Steam, I never leave it during this process, I can go from making a modification to looking up one's other people have made to playing and testing them out all within the space of about a minute and a half. The ease and accessibility that Steam has is by far one of its biggest trump cards, I could never navigate Desura or GamersGate like I could Steam and that's precisely the point, I know Steam better than I know any other digital distributor on the Internet, and although I may occasionally use them here and there, I know damn well that I will always come back to, and enjoy bouncing around Steam's menus a whole lot more than trying to maneuver myself through their labyrinthine-esque menu system.
Steam is just more appealing to me in every conceivable way and because of that I'd much rather use Steam than IndiCity for example, and even if there was a title on IndiCity, or Origin or GamersGate that I want that wasn't purchasable from Steam, you know damn well I'd only be using the competitor's service for that reason and that reason alone. If it were to launch on Steam there'd be no reason whatsoever to use the alternate cerise and this is where games like SimCity (the newer one) and other exclusive titles get off using ridiculous DRM services, they can abuse this sort of thing because they're using their own software and not Steam, so evidently the only big buying point that there is to using a competitors service is if they're being selfish bastards who want nothing more than to take the rest of money and leave me stranded with a game that's almost constantly broken and costs twice as much as any game twice as good as it.
The only reason we choose to drink Pespi is because the store we're in doesn't sell Coke.