I'm gonna do it so don't hate me. I'm gonna compare overwatch to Tf2.
TF2 still gets 50,000~ players a day, 12 years on, this is not a significant decrease from the numbers it has been getting since day one.
A huge reason for this is because of how incredibly deep the game is, every aspect can be mastered, no part of the game is just how it's presented. For example demoknight is a fantastic subclass which lets you tank damage and charge enemies for critical hits at the cost of only having a sword. but if you equip a sheild called the "Tide Turner" you open up a door to a skill called trimping, which when mastered let's you literally fly across the map, those who are trimp master's put hundreds of hours into the skill alone.
OW got really boring to me quite fast because it never had this aspect, it's a great game don't get me wrong, but it's a different game, it got repetitive fast because it didn't have depth in comparison. Instead of having these incredibly deep, niche skills, it just has a lot of different heroes, who's abilities are rather surface deep in comparison.