We lost 70% of our player base half a year in

[Valkyrie_Lenneth]

You posted a screenshot as evidence and you're not even reading it correctly.

First off, you're looking at peak, and not average. It's not a 70% decrease, it's actually about half as much as it was at expansion launch, or about the same as the month before the expansion launched.
Expansion came out in December, which caused the population to increase by 108%, or doubled, from the previous month.
Population decreases over the next few months as people finish the story and what content is available so they either take a break or log on less often.
6.1 releases in April and population jumps back up 40%.
Population stays around the same in May due to new Ultimate fight.

Pretty much this.

It's just another troll topic.