I think the underlying premise of the OP is wrong. Considering incomes, generally they increase with age. Using google (yeah I know that isn't always reliable) the average 16 year old makes $32,500 vs at 60 it's $62,000. However, teens have near zero in bills while older adults save at much higher rates. Most economists I follow consider 40 to be a break point. That is why "most" products are targeted at the coveted 14 - 29 year old. I would like to raise 2 additional thoughts I rarely see discussed.
1. Demographics - While I don't want to take time to cite all the sources, and some I can't. Working for a large corporation I have gotten to see the trends in our marketing over the past few decades. We generally market to a more mature audience, but realized roughly 20 years ago we needed to transition from Boomers to Gen-x, however due to simple numerical differences between each generation took a very slow approach. We "needed" to hold onto Boomers while targeting GenX. Therefore while we subtly marketed younger but made choices to avoid ruffling feathers. Now the Boomers have aged into retirement you would think we would more heavily target GenX, but the numbers have reversed, we "need" to get Millennials more than GenX. This has caused our strategy to change dramatically. IE Boomers and Millennials have very different tastes and GenX barely matter.
2. The Pandemic - while no one wants to remember that mess it changed people's habits. With things shut down and fewer ways to o spend money people had more available capital. A vast number spent that capital to find leisure at home. It was one of the biggest booms for gaming, PC, TTRPG, and board gaming alike. When it ended and people could go outside again those markets equalized, or busted. A number of those sectors are laying off, downsizing, or even closing shop. My only point is that 2014 could be as "popular" with the average "gamer" and still drop sales dramatically, which would appear to many that 2024 is "worse".
Lastly, it is just a simple approach to the market. Even if Mature Gamers (don't like the connotation of Gronard) spent equal to younger they aren't going to be targeted nearly as much. First most Mature Gamers started years ago and likely know very well what they want from a TTRPG, most (as polls here suggest) made up their minds when 2024 was announced. Second it's just a matter of age, if they "hook" a Boomer they may get a customer for 20 more years, but get a new GenZ, Zennial, they likely get 50+ years of spending. If I were them and a million to spend on marketing it makes simple sense to focus on gaining a few 1,000 new kids than retaining 100 Mature Gamers. Hence, the new focus of the 2024 DMG.
TDLR: Gronards (Matur Gamers) matter just far less than they would like and will (over time) spend far less than those darn kids. Also the pandemic screwed up the math and likely "ballooned" 2014 sales leading to a likely downward trend for 2024 (even if "popularity" is equal).