Correlation does not equal causation; the number of players naturally decrease with time, even with the release of new content. The expansion significantly renewed the player base, owing to new content and changing of internal systems (which the vast majority of players have hailed as a change for the better, debates over the Auction House aside, which wasn't actually an expansion change), and Seasons keep the player base spiking at regular intervals.
ARPG's have always been this way, and it's not surprising to see drops after content has been out for awhile. It happened to D2, happens to PoE, and every other game of the genre. Regardless, this isn't the place for such a discussion.
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The main issue with the shorter Enrage window is that there are simply too many buttons to use in too short a time. Obviously longer enrage is always better, that's a given, but when you have 4 abilities to use and only 2 GCDs to do it in, the rotation gets muddled.