I've bought more than a few themes at Envato. Some were simple and some were mega themes; some themes and support have been solid, some have been exasperating.

But I think there are two reasons not discussed yet that could be causing a drop in sales.

It used to be the case that someone who purchased a theme at Envator could post tech support questions in the theme comments and potential buyers could evaluate the theme and author by his or her replies in solving the problem. Now most authors ask for private messages or they tell buyers to post the question in their forum. All potential buyers see are the problems as they're posted, not the solutions, and it adds to the hesitation to buy. Let's face it, not all buyers at Envato can fix broken code. ;-)

An even bigger issue is the fact that so many theme authors — including best-selling ones — use and customize premium plugins they don't keep updated very well, such as Revolution Slider and Visual Bakery. If a buyer replaces the author's custom versions with the original, things can break, including author support for issues that arise. I'd be curious to know how many of the multitude of WordPress sites that were taken down by Revolution slider hacks were attacked while buyers were waiting for theme updates with fixes.