I've always thought of D&D's lifespan as being several different games. Note that I prefix my version numbers with a "v" to distinguish them from the edition numbers assigned by the publishers. So, "4E" is the game published as D&D Fourth Edition. v4.0 is my versioning.
OD&D is it's own version.
OD&D (1974) --> v1.0
Greyhawk (1975) --> v1.5
Greyhawk was a significant expansion onto the original D&D, eliminated the Chainmail rules for combat, and added a fair bit of complexity to the rules including spells over level 6, etc. While Blackmoor and Eldritch Wizardry can rightly be called just splat, Greyhawk is a real revision and expansion of the base rules.
Similarly, M&M D&D is also it's own version:
Holmes (blue box) D&D Basic (1977) --> v2.0
Moldvay/Cook D&D (1981) or D&D Basic/Expert --> v2.5
Mentzer D&D (1983-1986) or D&D BECMI --> v2.6
D&D Cyclopedia: 1991 --> v2.6.1
Holmes/Moldvay/Mentzer/BECMI/Compendium D&D is kind of a continuation of the feel of OD&D, but it's still a much different game. Moldvay changed a number of basic rules from Holmes, such as revising ability scores as I recall, while Mentzer was essentially just an expansion. The Cyclopedia was just a reformatting of Mentzer. Unfortunately for it's fans, M&M D&D became the OS/2 of the D&D line. I'm still tempted to bust it out and play it, but there's no way I'm going back to negative AC.
AD&D, on the other hand, has followed a pretty clear
tick/tock style development cycle.
AD&D 1e (1977-1979): v3.0
Yeah, I call it v3. It's my list and I pick the numbers. If you want a reason it's because Holmes came out before the AD&D PHB. I don't count the later printings of the PHB, MM or DMG as a new edition. AFAIK, except for Deities and Demigods the books were just a reprinting. The only change was the cover art. You can call it v3.0.1, I suppose. You might think it's strange to have versions of v2 released after v3, but, hey, that's what TSR decided to do. You know why AD&D didn't get revised sooner? TSR was busy with M&M. Unearthed Arcana was basically just splat.
AD&D 2e (1989): v3.5
Yeah, I said it. 2E AD&D didn't change the rules enough to be considered a new edition. This is a revision and simplification of the [often absurdly complex] original AD&D rules. Almost everything is seamlessly 100% compatible, and most of the "changes" were rules being dropped outright or moved to the DMG in order to make them clearly optional. The biggest change was the shift away from game mechanics being almost entirely unknown by the player. AD&D 2E is the first time the rules were significantly
reduced instead of expanded, but it's still the same version of the game. Playing AD&D 2E felt the same as AD&D 1E. All the Complete Book of Underwater Basketweavers crap could have been done with 1E rules, and the reorganization of the classes and spells is no more than D&D 3.5e did to 3e. So why didn't they call it AD&D 1.5e? Because D&D X.5 is a stupid name tied to a modern fixation on software versions.
AD&D 2e, Revised (1995): v3.5.1
The core books didn't change at all in 2e revised, except. They expanded by adding Player's Option and DM's Option, but since everything was explicitly optional, the Option books are really just generic Splat once TSR ran out of Complete Book topics.
The remaining editions under WotC are pretty straightforward, because, well, WotC is run more like a business and less like the back of a station wagon:
D&D 3e (2000): v4.0
D&D 3.5e (2003): v4.5
[Pathfinder (2009): v4.75]
D&D 4e (2008): v5.0
D&D 4e Essentials (2010): v5.5
4e could truly be v1.0 of a different game entirely, as the rumors that it was originally supposed to be the new version of the D&D Mini rules suggest. Still, that's not how WotC published it. Every time I think about 4e, though, I think that the single worst business decision that WotC has ever made with the D&D line was alienating Paizo Publishing.
D&D 5e (2014): v6.0
So, looking at that, you can see that all editions of the game have had major expansions or significant rules revisions within a few years of being released. The only exception is the amount of time between AD&D 1E and AD&D 2E, but AD&D 1E took three years to write and develop! Looking at the timelines, yes, we can expect an expansion, revision, or extension at any time.
