Sorry have no professional experience on RTS’s. Having said that perhaps this can open up the debate a little:
It would depend on deeper design decisions right? But from a high level I think what you are trading is complexity for content and these days people tend to err on the side of avoiding content. In what way do you see the game being less complex/easier for the story game?
I played myth, it’s a great game, lots of great ideas. I think ultimately they traded programmer time spent on opponent A.I. and balancing for lots of simple but interesting story based units when compared to a game like Command and Conquer (Warcraft3 has a great story mode too, so doesn’t make for a great comparison imo). They would also have been required to get their level designers focusing on pretty story levels more than highly balanced skirmish maps.
What is nice is that in Myth they don’t have to worry about unbalancing the design with one or two bad decisions. In a story game, a poor unit choice is something that affects a small part of the game (any time the show up in the story). In an RTS game it’s always there (past the tutorial levels).