I take no joy in sharing this opinion, but I think Deadlock is going to be extremely popular - perhaps not enough to dislodge the autistic clickers who already play DOTA or LoL, but certainly enough to establish itself in the wider market. Given how absolutely fucking abysmal the majority of games that are competing with Deadlock are (MOBAs, Fortnite, etc.), Deadlock's bare minimum mechanics for each character will make it stand out.
First of all, it's a Valve game made in the Source engine and that means doesn't have the "held together by chicken wire and duck tape" feel that most UE4/UE5 games have. Aside from the placeholder assets, there is a significant degree of visual polish here: I picked the character Viscous, who is made of translucent green goo, and saw that every little globule on his body applies a refraction effect to the stuff on the other side of it. The goo jiggles dynamically in real time, and generally looks really good. I can't think of anything like this in any MOBA or Fortnite.
Second, the character mechanics have the absolute bare minimum to be considered functioning video game mechanics, which automatically puts them miles ahead of any DOTA or LoL character. Viscous has an ability where he creates a big goo fist that punches out of a surface, knocking away anything in range. He can hit himself with it, he can position it on any wall or floor, and once he upgrades it he can have two charges. This makes the ability very versatile, and obviously open to combo with other characters. The simple fact that your opponent standing next to a wall gives you an additional option is far, far more fun than any top-down RTS-controlled MOBA.
Third, and most importantly, the game is a total information overload due to the number of items you can buy for your character: thus the "build order" autism the Blizzard inflicted upon us with Warcraft 3 is still alive and well, and your gameplay decisions in the moment will still remain secondary to the decision you make equipping your character with invisible items that drastically change your abilities and let you teleport across the map. If you play good video games it might seem like this is kind of shitty, but you have to understand the POV of the average subhuman who plays MOBAs. The volume of information functions like a backdrop that lets people invent whatever truths they want, allowing them to soften the blow of each loss. That guy didn't kill me because he outplayed me, that guy beat me because he's running Item X which gives him +20% damage resistance against bullets and my character uses bullets for everything.
The final point of allowing players to shrug off their losses and not take accountability for anything they do is how every popular team-based game has become and remained popular. Deadlock is going to make Valve lots of money and switch the popular consensus of them from a corrupt, monopolistic storefront into a hard done by team of scrappy upstarts who just wanted to make cool games and had a bit of a tough time figuring things out.