Yeah, I was gaming on my PC for long before any game client existed. (I'll leave consoles out of it.)
I am saying this because for the first year or so, I just couldn't trust the concept of digital downloads for games. Seems silly now, but then games came on disks. In fact no difference, disk or digital download. That will only have meaning for the older gamers.
Steam takes up to 30% of a games sale value (often less) so I think that's a bit high. But companies still wanted to use Steam directly without passing the cost to gamers.
The competition has always been between big rich game producers and developers.
I really think it' still the same. A vendor (Steam, Epic, EA) just can't be monopolies, they dont make the games we play.
Epic started off by having exclusive access to a big game (aargg can't remember the name) for a year. The hatred in the forums at Steam were over the top. Epic also only takes 10% of the sale value. But due to there practices of locking a game to it's platform only they started out as the company to hate. Most people said they would wait a year etc etc.
A year later that game arrived on Steam (with a full year of patches). It sold very well on Steam. More copies than Epic. Big fail, annoy gamers at set up of the company
It sounds to me that Epic were far too aggressive in their first year or so. They are guilty of unethical exclusivity. But despite the 30% cut Steam takes, vs. the reasonable 10% cut Epic take, devs know now to sell on Steam (and others) but Steam gets them the most sales. Also Epic has free games so why are they not as popular as Steam?
They had, and have a bad business model. So many people were furious and waited a year.
As a result of their really agressive steps to get into the market it left a very sour taste. Moreover they still are not popular. No big change overall Still looks the same. That's bias of course, but it is well deserved.
Personally I trust Steam, and like the whole interface and services that they offer. And still I don't find EPIC a pleasant platform. I don't hate it, but I don't like the client. Simple as that.
EA have earned my refusal to never use them again. That's for another time but I lost cash and got no C.S. They are terrible, and many say so.
Steam hasn't tried to get any hard locked exclusives as far as I know. Developers are free to choose which (including all) clients they trust to sell there games.
Steam is now the go to place for most gamers, who on occaision may check out EPIC or the terrible EA (I had bad problems with them) for a better deal or just a free game.
They have the most, by far gamers because the whole experience is better. EPIC is the only company (as far as I know) that started off with exclusivity.
Steam can't help it if most gamers prefer them. But it's a natural thing for customers to buy anything (not just games) from a retailer that they prefer.
Valve had put enormous effort into developing and still do for their Steam platform. They have a platform that is great. Cant say the same for Epic
Okay, my post is BORING so to finish I will set up a dubious metaphor. 
In a big city, there are only 4 Italian restaurunts. They are similarly priced.
For the most popular one the experience starts at reservation (they never over book) It really gets going when in the place with a very friendly and polite atmosphere. They care about their customers and offer a top class service, not limited to just the food.
So customers feel real comfortable and happy. Probably makes for a good evening experience.
The other three don't have great, just reasonable service. There food was similar quality to the top one at first.
The others thought they could save money by hiring only a few staff, and didn't create a pleasing, Place. Daft business practice.
As a result the best one gets more and more business. They invest profits back into the business, coming up with new dishes because they can afford to have enough staff for"research." Eventually one of the four is running at a loss. No one wants to go there anymore when they get a better experience at the best one. Plus they keep customers interested because they inovate and never let the whole customer experience drop. It gets better and better. More new exciting food,, I could go on but not necessary.
Even if the best one caused ALL the other 3 to go bankrupt, you can't say they have a monopoly. The market is wide open to new restaurants.
The best one ends up being the only one customers want to go to. They continue to invest, and may even open a second or third restaurant with the Promise of the same service.
All this time the market has been wide open to other companies. They end up having the whole market in imaginary city. But it is not a monopoly legally.- they got there without tricks and offered what customers want. So people went/go to their favourite. In legal terms the market is wide open
That's not their fault. It's through hard work, innovation etc etc that makes customers choose them. Can't tell customers only go there once every six months, or force the restaurant to degrade the lovely atmosphere, even the great and varied food they have worked up to.
I'm not sure if the very dubious Italian metaphor restaurants works really, mmm. 
EPIC started off with a slap to gamers.(AAA game one year exclusive) They never stopped the minor tricks (a game that is free, but not really because only the base game is free. They have not improved their business. Most gamers have no reason to go there. They don't do the one year exclusives anymore (I think) as companies realize that it's bad for business, if they could still do it they probably would (opinion only).
They offer a sub par online store and don't change. Sure they do still offer some great deals (usually with a need to pay something in the end) Moreover for the last two years Steam have had some crazy good low prices. They have out done Epic at their own game.
Finally they sell games from many big medium and Indie devs).
Any monopoly would be by the actions of game devs. Not the vendor (Steam or other)
The market isn't locked up either.
Simply Steam have a good business model. Epic is becoming irrelevant, and possibly will shut down years a head. That's there fault. They have access to sell games from many big devs, but devs and customers avoid them more and more. That is not the fault of Steam.
Plus to say for the100th time, any monopoly would apply to the product, the game devs.
Not the vendors. People are free to shop where they want, and if any of these vendors are doing naughty stuff, it's not Steam.
Sorry about the length of this post, and the boring bits!!
