A tree is known by its fruits. I think it a poor strategy, regardless of how it evolved, because it is not a sustainable strategy and I think the whole 4e-5e "fiasco" sort of demonstrates some of the difficulties with it. I would also suggest that "the whole industry" does not follow this strategy. [/quote
Name a few games which have stayed 1st Edition for, say, the past five years, and whose publishers are still actively supporting the game. Now expand that to 10 a game with no new edition in 10 years.
Without a doubt, adventures sell slower than core books. That's a given. In fact any supplement is going to sell less than the core. But this does not make the supplements less important than the core. Each supplement helps to continue the presence of the core and, if it is quality, expands the appeal and thus base of the brand/game. Naturally supplements should be printed and produced in smaller margins than the core, but they should be an ongoing thing and though each sells less, if you do a good job, they should sell.
Do they sell enough to justify their development, printing and marketing costs, and support the business by contributing to covering overhead and profits? How many companies leapt on the OGL bandwagon to publish adventures for the biggest RPG of them all? How many were still in business selling such adventures when 4e was announced? Of those companies still in business, what portion had instead branched out to their own games, producing something (presumably) more lucrative than D&D adventures?
At the same time, it is to be hoped that the core is flexible enough to be used to provide a variety of supplements to suit a variety of taste, thus, and again, continuing the life of the core, which should be your green product. Supplements should never replace or so alter the game as to make the core unnecessary or undesirable, but they should stretch the core.
This also points to the advantage of the OGL, which allows 3pp to produce material to further expand the appeal of the core and reach an ever wider audience.
The clear advantage of the OGL is, of course, demonstrated by the substantial number of publishers who have adopted a similar model, and WoTC's continuation of the OGL through 4e and D&D Next, right? The fact ios that those third parties diluted the brand. While there were some gems, there was a lot of Sturgeon's Law evidence as well.
I'd actually go farther and say that the core should be flexible enough to support different games. After all, during the early days of 3e, WotC was producing Star Wars, CoC d20, and d20 Modern/Future/etc. simultaneously.
GURPS and Hero come to mind (sounds like Cortex as well), a game system designed in front of any specific game. The general systems have the advantage of supporting multiple genres, where a more focused game has the advantage of being able to focus its rules on emulating the tropes and feel of a specific genre, or even subgenre.
Personally, I think they should approach it in a manner that board games are sold. If you're selling say, Monopoly, you don't target it to sell a copy to every person who's going to play. You sell it with the idea it's going to be played by a group. If you make add-ons, make it with the intention it will be sold to the group, not one person. At the same time, I really think that RPG companies have gotten into the bad habit of thinking that if they sell you a copy of Monopoly, they need to make dozens of add-ons (Monopoly: Skyscrapers; Monopoly Token Pack MCXI - Cat, Hamsterball, and Monkey; Monopoly: Chance of a Lifetime; Monopoly: Train Tycoon; Monopoly: Prison Break, etc.) rather than try to get you interested in say, Clue. Go back to the days of having D&D, Star Frontiers, Boot Hill, Top Secret, Gamma World, Alternity and half a dozen RPGs out in field rather than sink all their eggs in one RPG where you have to make product for it every flipping month or go under.
How many of those games were in publication at the same time? Alternity was quite a bit later than those others, as I recall. Seems to me there was a bit o mechanics testing for 3e as well. Those games also got pretty limited support. I think, though, that Top Secret got at least one "new addition" and Gamma World has had several. In fact, the Gamma World proportion of editions to other support is a lot higher than ay D&D edition, I think.
The "numerous games with a bit of support, then move on" model was very much the '70s/80s model. Rules supplements came out on occasion. OD&D had more supplement support than scenario support looking at TSR, although Judges' Guild filled some of that void.
Y'know, diversify. I actually think that's one thing that has hurt D&D - it has come to a point where if something new (or two) isn't coming out every month for it, it's "dead". That's too much pressure. I mean, go back and look at the release schedule of AD&D (1E) in it's heyday. I seem to recall new rulebooks coming out like once every couple of years, and modules not much faster.
1e at its start had three rulebooks. I wouldn't call Deities & Demigods a rulebook. The new rulebook, Unearthed Arcana, was an event. As we approached the end of 1e, we saw the model evolve to present more rulebooks. By that time, TSR was, IIRC, pretty much out of the non-RPG game business (they did some board games back in the day, remember?), and I don't recall support for many other RPG's at that time either (the last big one was Marvel Supers, again IIRC). Actually, their biggest supported games were, realistically, D&D and AD&D, two editions running concurrently.
And in the flavor you can have Simpsons Monopoly, Tron Monopoly and whatnot, you keep the same base game but flavor your version for different tastes. Think of doing campaign worlds - FR, Dark Sun, etc. The big thing is, don't keep trying to support that campaign world. Do it, then move on to the next flavor of the month.
That was certainly part of the model, but more "put it out there, see how it sells and decide further support from there". Ravenloft got tons of support, Al Qadim not so much. That's not always a formula for success, though. Hero has noted that, while its Sci Fi genre book, Star Hero, is one of its best sellers, other supplements for that genre don't sell.
Finally, I think WotC should possibly look at having different versions of D&D - in the way you have City Monopoly, Millionaire Monopoly, Boardwalk and the like. Though these games can significantly change the look and feel of the game, they all have a similiar core - and they don't have to be supported as more than one-off variants. So, maybe we have some base D&D game (perhaps looking a lot like BECMI D&D), an Advanced version that models 1E & 2E, an Options D&D (mimicking 3E) and some option that mimics 4E (sorry, can't think of some witty name at the moment). Or just reprint the four (?) versions and round-robin products for them.
That's not too different than the theory of 5e, is it? One base, many optional modules. The question now is whether all the modules go in to the core rules (Hero's Big Blue Books model - and even that isn't 100% complete) or a base game with modules sold separately (sounds a lot more like GURPS, although Hero and GURPS aren't perfect comparables, being designed for multiple games and genres).
But, as has been pointed out a time or two, WotC will still have to select a base that it will aim adventures and other supplements at, doesn't it? Half a dozen modular systems will splinter the market as much as multiple editions all played at the same time.