To clarify on the above graphs:
Only sets with at least 10 pieces are included
Sets from theme "Educational" were excluded
Sets from sub-theme "Product Collections" were excluded
Some sets were manually excluded (e.g. staff gifts, tour sets)
I think the main problem is, just like Faefrost mentions, flooding of the market. Too many sets are released on a yearly basis, both as a whole and within specific themes. It's becoming difficult (especially for kids) to decide what they want. Even more so, impossible to collect everything released within a theme of their interest. There are now seven $100 sets within a theme, rather than one or two. There's simply too much and I believe the long term reaction will be that people simply won't bother.
I believe some themes are kept for way too long (Star Wars, Minecraft) and some rather new themes (Nexo Knights, Ninjago, Batman) have received way too many sets already and the interest for these themes will soon die. Same with the DC and Marvel series.
I think LEGO needs to go back to its roots a bit more (while still staying modern) - more sets around classic themes such as pirates, castles, boats, airports, animals, cars etc as well as SHORT-term and FEW sets around more niche, limited themes like current TV shows, computer games, movies and so on. The jungle theme is interesting and in the right direction, imo.
Technic, Architecture and Ideas are great addition for an older crowd, imo!
Perhaps LEGO sales numbers say otherwise, but it's clear they're facing challenges considering the lay-offs.