Good point - I wasn't buying stuff in 1999; 3e brought me back and really it was 4e (& Pathfinder accessories) started my whalish habits.Today, 35+ are very active in the market. But remember that the market research under discussion was done back in 1999. That's before the boom of 3e, and again of 5e. The diversity of 3rd party materials (both game rules and physical aids like custom dice and GM screens and all) didn't exist. The market was a different place.
The 35+ of then are the 55+ of today.
One huge source of expenditure these days is plastic minis; not a thing in 1999.