I was thinking of non-D&D games. Dungeons and Dragons would be hard to do under 100 pages because of things like spells, monsters, etc.
Let's take a step back. How about the Basic Edition (or BECMI series) of DnD?
From an entry-level, or new player perspective, you had everything you need to play in one box. A single booklet contained enough rules, monsters, and guides for both players and DMs to enjoy the first few levels of play. There was even an adventure module to get you started. Do we really need all 20-30 levels of play right from the start? I'd argue that you don't. Long term campaigns are hard to maintain. Many groups barely make it past a certain point. And most people may not care to play past a certain point. They want something more casual. So why make them buy more than they can use? It should be enough to get people interested, try it out, and decide for themselves if they want more. If you give players a good, solid and complete game before reaching its "end game" level, then let them decide for themselves if they want to invest in the "next" stage (or tier) of the game.
The 4th edition approach really failed in this regard. I wasn't even interested enough to look until PHB2 came out a year after its release. Why? Because the game felt like it was only half done. Where was the half-orc and the gnome? Where was the bard, the barbarian, and the druid? How could this be a current DnD game with those iconic pieces missing from the start? And why do I need everything to get my campaign to level 30 (or even 20!) right away? New game means I should be taking some time with the basics, which starts at the lowest level tier. Get me to level 10 for now and let me enjoy it. By the time my group is ready to move beyond (i.e. decide if we want more from the game), then we should see the next book coming out to take us into the next stage of the game.
It may be hard for some people to believe, but not every campaign or group needs to reach the maximum level of play to be enjoyable. There *should* be allowance for casual gamers to enjoy the same game, and many stay away from it because it is specifically catered to the more dedicated and hardcore crowd. You want new players in the hobby? Lighten the load for them. Just because the game system is capable of carrying a group for months and years in the same storyline doesn't mean that should be the expectations of all.
Wizards/Hasbro has this idea that many people don't play the game because they haven't marketed it more towards them. Marketing has nothing (well, almost nothing) to do with it. People look at it and see dozens of books around a game table and think to themselves "Do I really need to buy all this just to play a game?" And gamers can argue or explain that all they really is one or three Core books all you like behind your stack of "optional" material. You know they're thinking to themselves, "So why don't any of you have just those books??"