Parmandur
Book-Friend, he/him
I'm not sure why this is supposed to be meaningful. Buy what you think is useful or will enjoy. Are there really that many people who buy everything?
The downside to me of lots of product like in 2e is that I have a harder time evaluating whether a product would be useful or interesting, not that I can't purchase it. The same problem exists with the flood of product on DMs Guild.
The only gaming system I almost bought everything for was WEG Star Wars. I found it interesting enough to keep buying new products. The products had good production values and great art (from the LucasFilm archives), at least until the final few years when the trade dress changed from product to product and quality plummeted. None of my friends were buying it after the first few products (important as in High School, for example, we got a complete set of 1e AD&D books by having one person buy a book outside the core three. My purchase was the Wilderness Survival Guide).
In 3.x and 4E, I bought one book per edition: the PHB in both cases. Not that other books didn't appeal, but there were too many, so I went with buying none due to indecision.
Now, I'm on track to get every single 5E book, though I'm still missing a couple due to the speed at which they are releasing them...