The thing is - Multiplayer rts gamers are the minority - 3% of all customers go online the rest moves on after the campaign and some skirmish. The problem of course, is that SP Gamers (unlike me) do not go on a forum to post about it - They simply move on. (Or have you never wondered why Total War sell this well? In that people really do not move on, mods new campaigns new nations.. its pretty complex sp xperience) ;P
I for one have played any campaign of any rts, and have not played a single multiplayer session for years.
My last rts was CnC 4 which rocked - The Campaign was awesome (as usual) | Warcraft 3 also had a very good campaign (even has RPG maps ,p) and starcraft had a superb campaign (Back in the days, and unlike that horribly boring Total Annihilation or Supreme Commander (which was even more boring))
To say that rts games campaigns are for training is the silliest thing i ever read. They are a *campaign* the opposite of pointless online skirmishes!
If you'd want proof for this, check how many people bought Sins of a Solar Empire - And check how many are still around (Would say, less than 1%) to play it. Sins was imo the worst rts in recent times as well - Imbalanced boring and entirely pointless and no campaign. ^^
(Not asking for a campaign in AI War though ;p)