So, it is possible for them to have a satisfactorily large amount of content, but only if they embrace randomly generated maps and enemy compositions like Diablo 3 and Path of Exile. This allows the game to continue to feel relatively fresh since you're seeing a large variety of landscapes and might even face difficult enemy combinations that you wouldn't normally see. I really hope that they go this route.
I don't want to assume how far you got into Diablo 3's end game, but it sounds like you never really challenged yourself in high torment levels while still getting decent gear or in Greater Rifts (GRs). For those who didn't play Diablo 3's expansion, GRs are a theoretically endlessly scaling series of dungeons that just keep getting harder and harder. If you tried to get to higher levels, you'd get to points where you'd have to be playing very well to continue since there wasn't much room for mistakes. Depending on the quality of your gear and your build, you'd have to be managing your cooldowns well, staying out of harmful effects, managing enemies to fuel your abilities or power your buffs, time your buffs for maximum effect, and generally not make any mistakes since you could potentially die instantly if, for example, you let a buff expire even for a moment while in danger. Although the technical skill required in Diablo 3 to click on enemies is perhaps not as high as accurately targeting and shooting, there is still a lot of timing, situational awareness, and positioning required that separates good players from mediocre ones.