I mean, look at the great online games of yesteryear - say, Quake for example. Quake's mechanics are fundamentally simple. Run, jump, pickup stuff, shoot. There's only 9ish weapons to figure out in the average Quake-like game, and in most cases the behavior is "point at what you want to die and click the button". Learn the maps so you can control the pickups. The hardcore players will have to learn to exploit the bugs in the motion model to get a little speed edge, but that basic stuff is all there.
And in a Deathmatch game, you know what happens if you suck? You die. Over and over and over and over again. But since it's a free-for-all, there's no teammates you're letting down. Nobody screaming at you about how much you suck. And even in a teamgame, the teams are larger so one weak player can fade in the back. And none of his teammates are watching him suck and screaming at him, they're all too busy playing and can't see what he's doing through walls. If he drops out, nobody cares.
But you can't say Quake et al are low-skill. See also RTS games - RTS team-games face many of the same problems as DOTAlikes, but in most RTSes the 1V1 matchups reign supreme, and FFA is also a popular option. In a 1V1 a newbie will get utterly annihilated repeatedly and there's the same bad slippery slope of DOTAlikes, but nobody will hate the loser like you'll see in a teamgame where everybody just watched one idiotic ally spend the last 25 minutes ruining their fun. Plus, RTS games at least will give you the dignity of a quick death unlike a DOTAlike where you'll still spend several minutes watching the towers go down before the victor is declared.
You could make a super-simple DOTA-like game with only one hero to choose from and super-simple combat mechanics and you'd still see all the cultural and mechanical problems you see in modern DOTAlikes. You'd still have players screaming at the newbie who's accidentally feeding. You'd still have players hurling verbal abuse at each other while they get pointlessly pummeled because the game is functionally over but doesn't have the dignity to end even though the leveling has made it completley moribund.
It's not about skill, it's about a long-form team-game with bad slippery slopes. A low-skill game with that format would still be cruel to newbies, just like high-skill games that avoid that insanity are newbie-friendly (see any twitch-FPS for example).