I think BRP and Fudge are the few systems that have aged well since their inception. I attribute that in part because of their flexibility and in part because they either started with, or eventually received, open versions so that folks could tinker with them.
@craigmaloney Based on the retroclones: OD&D and B/X aged very well, and are better than anyone thought at the time, AD&D 1-2E poorly to the point there's just one serious retroclone of each, 3E & 3.5E are survived by Pathfinder because no "fan" could maintain them but there's some demand for that nonsense, 4E is abandoned and loathed, it remains to be seen if anything of 5E survives.
@craigmaloney Tunnels & Trolls: Deluxe (and now the Japan Mini-Rules, which are almost their own edition) are different but still compatible with T&T 1E from 1974, and there's a good supply of new work.
Fighting Fantasy -> AFF -> Troika and a bunch of other dicechuckers are still popular and also changed only in the support materials since 1982.
@mdhughes I'm not as familiar with Tunnels & Trolls. I knew as soon as I posted this that someone would highlight something I missed. Same also with Fighting Fantasy (which, aren't those the series of books that could support solo-play that also got ported to a larger system, or am I thinking of something else?)
@craigmaloney Yeah, FF started as just solo gamebooks, then they made a borderline-normal RPG, and later a much better AFF 2E. Even so, you can pretty much take your Warlock of Firetop Mountain hero and play them in AFF 2E with minor additions, or vice versa.
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