Over recent years, I've noticed a lot of villains that used to fit the arch-enemy role have kind of segued out. Different factors include them turning more good than bad, other villains taking over the role, etc.

Who are some examples of villains that can no longer be seen as an arch-enemy?

Magneto is the first person to come to mind for me, and I would argue he's the definitive example. Even back in the day, he tended to flip flop but at this point, for every story where he's been the villain, he has 5x as many stories where he's a good guy. I guess the problem is that given his character, it really wouldn't make any sense for him to stay bad. His cause has always been noble, and he's a very human character, so it's almost unrealistic for him to continue to be bad.

Sabertooth is another example. His rivalry with Wolverine seems to mostly have been left in the past.