Warcraft III was also released in 2002 (I remember "fondly" what happens when you release an RTS right between AOM and WC3)... how many PC RTSes have been published since then? Possibly 50, of which certainly more than 10 good ones. Can you point at other RTS mod success stories? I think everything is disproportionately skewed towards WC3 mods.
Releasing a sequel every year gives more than enough time for modders - you don't need more than a couple of months to get familiar with the game enough to make mods, much less maps. (The very deep gameplay/ballance aspects of games like SC and WC3 that are still unexplored fully after, say, a year, are a different type of knowledge, not the same type of "knowing the game" you need to make something like DOTA.)
You did pay lip service to the "the industry is straggled by a few huge players" mantra earlier in the thread, yet with the above words you seem to favor a model where there is something like one major RTS in the world, and new versions are released once in every Blizzard aeon. Do you really think a small developers stands a chance if they release one game in five years?
Look back at the history of modding and map making, it's nearly always been towards the one giant in each gametype. WC3 for the last 5 years in the RTS realm. I don't know a single series in history that's had great mod/map support that releases a sequel nearly every year. How is 1 million users (constant, ones who played near daily? Doubtful, maybe 1/10 of that) spread across 4 games good? It's not. You have people who never move on, people who do move on. People wanting to make maps that don't, those who want to make maps who do, etc, etc. You fragmented your market like crazy. Expansions after a year isn't so bad as long as the non-expansion and expansion players can still play together in some ways unless you've sold a massive amount.
Modding is not the answer to "microtransactions" in their current state on consoles. Modding will never really be the answer to it at all. However, in the current place that microtransactions are at is no longer how they started out. In the past (and currently in nearly all non-console cases) it was an alternative way to make money from the game past initial sales. Instead of subscription for your MMO you charge very little for in game items, the amount paid was small. Now though map packs and stupid non-gameplay effecting items are released for ridiculous prices. Where are the days when you got maps as part of patches? Is it really that expensive to release the maps as free to continue to spur new purchases or simply support and make your player base happy?
I don't at all believe small developers should be targeting the mod crowd in the first place. It's down right crazy to expect you'll have a healthy modding community on a very small game for the most part. A small developer should fully concentrate on making something original (most don't, most small projects are just poor knock-offs of larger games). You can't expect to get into the market with the same game. You can release a game that is similar but it must have some aspect that is new, some aspect that is accessible yet has major gameplay changing factors. It very rarely happens. Being a small developer is extremely difficult for more reasons than just you're the small guy. Though I don't at all believe support should be given to small developers simply because they're small.