Gamers have grown up. They're in their late 30's now. They have more disposable income, but less time. I know that occasionally I don't finish a game unless I get really invested in it. It becomes difficult to revisit a game after you've put it down for a length of time.
Plenty of recent games like that. Grand Theft Auto, Just Cause and Saints Row to name just a few, all on console and/or PC. All open world and all allow you to mess around endlessly in the game world.
It can be a 40 hour playthrough, at least, depending on whether you go after every little side mission (and... organise your weapons / armour). 'Easy' depends on the difficulty you set and how you play your games usually. The story's pretty entertaining and the depth of the lore in the game could make it seem deep, I suppose.
Sounds sort of like the open quests from Warhammer online. Show up in area, help out, get loot bag. True, there's a ranking system which means if your efforts weren't good enough you won't get anything immediate, but you still earn points which raise your rank in the chapter and (eventually) enable you to pick up useful, class-specific equipment.
Steam sometimes comes with third party DRM. You'll note ACII on Steam has the same restrictions. I do agree that it's a very convenient platform by itself, though.