Back in the final days of Northrend, (other) bloggers were worried that
the end was comming. Way back then, players would need 6 glyphs (if I
recall correctly) per spec - so a total of 12 glyphs. Your average
players changed glyphs ... less often than a 8 year old voluntary
Long time scribes remember glyphmas, where we got to have 9 glyphs per spec, and could permanently learn all glyphs (roughly 30 per class). This followed by a boom while everone was getting to 85. We know that generally demand has fallen from those wonderful days. Established, fully glyphed lvl 85 toons simply have no reason to buy new glyphs. But all is not lost.
This graph from MMO champion shows a steady stream of new level 85 characters, that has essentially been stable for 4 months now. Spikes over the weekend, and troughs on Mondays (no one likes Monday). While the graph only shows level 85, to get this level of consistency means that you must have a consistent stream of levelling toons. They also mention a brief dip when 4.2 came out.
Seriously - from a demand perspective, this is as bad as it will get.