You could start with demonstrating why Avant Garde is a poor choice for body copy which will affect the brand’s communication. Type with strict geometry, extreme proportions, very small apertures, and various uniform/indistinguishable letter shapes does not make a pleasant or effective reading experience.
To get back to @Nick Cooke’s original question, I crunched my MyFonts affiliate numbers. While your experience as a foundry may vary, maybe these figures can shed some light on the overall sales trends.
The average price of sold units (combining both singles and families) has fallen sharply over the last three years. It was stable for at least five years, hovering at $40 throughout 2008–12, but dropped to $38 in 2013 and again to $36 in 2014. In the last quarter of 2016 it was at $34.
Unfortunately, I don’t have more specific data so can’t pinpoint a single cause. As others have mentioned, there are various suspects: decreasing family sales, the Monotype Library Subscription, and other notable MyFonts trends during this period that lead to cheaper fonts, like heavy discounting and frequent flash sales from Monotype as well as third-party foundries.
And of course we have to consider outside influences as well, such as cheap and free alternatives from Typekit (library subscription) and Google, whose collections and influence grew significantly during this period. Webfont services also gained traction in 2013–14. Despite these alternative sources, my individual affiliate sales at MyFonts actually went up during those years — it was the average price of each unit that fell.
In early 2015 units sold began to decline steadily and have not recovered. This was many months before the introduction of the MT Library Subscription, so I wouldn’t blame it specifically.