I started my RPG hobby back in 1998 with the Swedish fantasy game Eon, which was only two years old by then. The core rules were 200+ pages if I recall correctly, and it already had a couple of source books with things like detailed information on a couple of countries in the setting and expanded magic rules.
Now Eon celebrates its 25th anniversary and the game is something grander than any other Swedish RPG. Back in 2014 the 4th edition came out. The rules were radically different while still maintaining the same feel and more to the point being better adapted to the most common play styles. I think they included information on how to convert old material, but I'm not sure. The core rules were now 400 pages and included deep ties to one of the lesser documented but more popular countries in the setting as a starting point. Notably it didn't include magic rules.
I'd already stopped playing the game some years before and sold off my collection of a dozen or so source books. More had already been published at the time.
The team that's been behind Eon since the new version came out is ambitious and talented. They've produced a huge number of source books and adventures and continue doing so, including a series of dedicated books for the 19 types of magic (yes you read that right, although it's technically 21 but the last two are reserved for gods and demons). Many of the new books have addressed and fixed inconsistencies in old material, and made the big setting with its varied cultures and customs more nuanced and believable. They're now planning a 5th edition, which will be rules compatible but focus on a different country as introduction to the setting.
Eon has a huge and loyal fan base, much of which have been with the game for more than a decade. It's a game that's fulfilling its potential and vision as a detailed and expansive culture game. Yet I can't for the life of me work up any enthusiasm at all for it.
The biggest and most complicated game I run these days is Labyrinth Lord. It's a more-or-less loyal clone of Mentzer's B/X rules for Dungeons & Dragons from the early 1980s. That was one of the Basic editions of the rules, named so because TSR wanted to bring an Advanced Dungeons & Dragons to the market. Labyrinth Lord is 133 pages and contain everything I need and a lot (and I do mean a lot) more. For example I've never needed the rules for naval or aerial combat, neither for overland travel and wilderness adventures. If I ever get to running a full campaign in it I'll probably use the latter two, but they're not long chapters. Some very long chapters that I've equally never used is monsters, dungeon master advice (including a detailed guide on making dungeons), magic items, or treasures. Those together might even be half the book. I haven't checked.
Labyrinth Lord has no setting. That's a gift and a curse. A gift in the sense that there are no source books establishing a version of the setting which a lot of players would assume is canon. A curse in that the DM or group has to build a setting themselves.
But you know what? I could never be bothered to read a source book nowadays. I can't even imagine reading scores of them. Neither could some of the authors of old adventures to Eon, it seems, because the only few adventure modules I read back in the day blatantly broke the core rules and canon material.
Today I wonder who likes a game like that. Or rather who picks up a game like that to start playing. I can definitely understand those who've digested a book at a time as they've come out, but I find the whole idea of all those books even existing as extremely intimidating. In 2019 I ran a short campaign of Mutant Chronicles 2nd edition, having only read the parts of the core rules I found relevant. All of my players had read four or five (or however many there were) source books about the different factions and other things in the setting. Through the campaign I ran into a few reactions of "This could never ever happen because book X says this!" even if I'd read the sections in question and found them to be very generalised and open to some flexibility and nuance. Because those books, broad generalisations and all, were considered sacred canon.
I believe there is some sunk investment fallacy involved here. People who have spent time and money on these books want them to matter greatly, because that's a return on their investment. That's definitely an understandable position, but it does mean that I as a person with little interest in spending time and money reading these - potentially tens, dozens, or scores of books - am at a disadvantage and feel discouraged even picking up a game with that much legacy and canon.
A funny side note is that I actually know one of the authors of the rules and core books for MC 2nd ed. I explained my campaign ideas and interpretations to him, and how I envisioned the different factions as more dynamic and nuanced than the generalisations in the books and he responded with "Awesome! That's exactly how I envision it! That campaign sounds epic!"
-- CC0 Björn Wärmedal