MoP didn't have infinite progression like Mythic+ though, the content for timed runs were generally static while your character simply progresses in power and is able to beat it faster and faster. The difficulty remains the static.
Mythic+ rewards and challenge grows, and the timelimit gatekeeps the rewards. Therefore the meta suggests gatekeeping players in order to maintain a certain level of progression.
As I've explained, Greater Rifts generally don't have this same problem because the rewards of Greater Rifts is tied to the progression regardless of time limit. There is no real punishment for going over the limit other than having to invest more time. There is much less of a need to gatekeep who participates in the GR runs since the loot is rewarded as long as you all beat the boss.
Diablo 3 is designed in a way where roles are also different and you can literally have 'Zero DPS' classes in the game who do nothing but provide support, and get 'carried' by the DPS players who you are intentionally buffing. This is actually the intentional meta for high level GR group play; you have a Scouter who runs up ahead and gathers Elite mobs together, a ZDPS support who keeps everyone alive and crowd controls, and 2 DPS who melt down everything else. But the content is absolutely doable with 4 DPS or any combination of roles and classes (barring all ZDPS). WoW is designed around the Holy Trinity, which has a much higher emphasis for the Tanks or Healers dictating who gets to do the content. This is a bigger symptom that also affects other LFG content, like LFR and Dungeon grouping overall. This all contributes to the toxicity and expectations of players who are using these systems specifically for power progression.