Not that its a bad definition, but I would say Fate (especially Accelerated) fits that definition.
In a very simple stripped down form, FATE probably is a rules light game. The reason that I don't think it is in practice a rules light game is precisely the way you say it bucks that trend.
There's a ton of additional settings and modulations on the rules...I think this is because Fate has the peculiar option of Extras and is easily modifiable in other ways as well. So, you can play any genre/setting of Fate right out of the box, but if you want a little more suggestion for exactly how to pull something off, you can buy a setting book that has a bunch of "pre-brewed" mechanics in it already.
And all those extensions to the rules are exactly why it doesn't fit the definition I gave. Those extensions add complexity, increase the scope of the game, try to simulate something specific (the genera conventions of a particular setting), and make the more than a single game book. They don't quite make FATE rules heavy, but they keep the game from being rules light.
There is a similar thing with my own homebrew rules light system: SIPS. The standard game of SIPS is about as rules light as an RPG can be, because it was designed to be played by 5 year olds. But, "SIPS for Hogwarts", is rules medium because it's designed to directly simulate the conventions of the Harry Potter universe. Gone is the very simple super-power rules, and in its place is a more complex system of magical mastery. One reason for this is that it isn't all that interesting if every SIPS character has the superpower, "You're a wizard, Harry!", or some variation thereof.
I wouldn't say that supporting long term play is a necessary condition for something to be an RPG.
I never said it was. What I said is that if your game doesn't support long term play, then it limits the ability of your game to be successful. Your game becomes something that groups do as occasional one offs. If your group plays a rules heavy campaign for 20 sessions, and then plays some Indy rules light game for one session, before returning back to the rules heavy game, you're penetration of the market is going to be inherently low.
Nonetheless, I would agree that there is a segment of the role-playing market that revels in its mastery of KOTOTU (Knowledge Of The Otherwise Totally Useless) and that rules light games wont appeal to them because there will not be all those listy things from which they derive their jollies.
While I agree with you, that is both an uncharitable way to describe the problem and only one example of the problem. An RPG which is composed of a collection of different minigames supports inherently multiple aesthetics of play. One of those minigames might be chargen, and a rules heavy game can have complex chargen systems which make for compelling minigames in their own right, often leading to people who enjoy that aesthetic of play, playing that minigame as a separate game in its own right. But a rules light game inherently supports few or even one minigame and thus really is supporting few or even one aesthetic of play. This inherently makes fewer potential players happy, and makes almost all potential players less happy over time. Variaty is the spice of life.
Demonstrably untrue (at least for Fate). Starting and running longterm games with little to no prep is a common occurrence for Fate players.
I'm not going to turn this into a FATE bashing thread. To each thier own. There are a lot of things I admire about the FATE system and its contribution to our knowledge of game design. Let's just leave it at that.
Of course, I've said that I'm not sure I personally count Fate as rules-light, and even in this thread it seems that consensus is that it hovers near the border of light/medium. So perhaps it escapes your concerns by that means.
Yes. Exactly.
Don't get me wrong. I think there is a place for rules light games on the market. There are number that I want to play and I hope some time to get the opportunity to do so. I just don't think that there is ever a way for a rules light game to be successful in the way D&D, VtM, GURPS, Deadlands, and so forth have been successful without evolving to something that isn't a rules light game. Arguably D&D and VtM started out as rules light games (or rules medium) before evolving into rules heavy games in response to their own success and the demand from the players for more rules.
What I'm saying is that if you are planning to print a truly successful 'rules light' game, you probably need to be closer to the 'rules medium' end of the spectrum and leave room for the system to evolve to a more rules heavy system by plugging in additional rules modules.