That is an excellent point. My last F2F campaign ran for 19 years, and generated very few sales from the group; nearly all the purchases were by me, and I am not an avid collector in any sense of the word.Even pretty good increases in the number of new players only affects total sales to a certain degree. If you want to play a new sport, you usually have to buy some equipment. If you want to learn a new craft, you usually have to buy supplies and tools. If you want to play a new video game, you usually have to purchase a copy of it (or some other form of access). Not all players have to buy ttrpg books to get into the hobby, though. In fact, an awful lot of them don't, or only make one or two purchases (ex. a player's handbook). Most ttrpg books sell to GMs, who only make up a certain percentage of the total hobbyists.
Since shifting online, I now have two campaigns, each of five players. None of those ten have made any RPG purchases for the campaign they are in. I purchased two books through Roll20. That's it. All the adventures are from free sources o the Net (not piracy). Likewise, the maps are drawn from Pinterest.