First and foremost, in order to win an online game of Sins you have to employ a monotonous strategy in order to win. This means rushing to tech to certain units and technologies because they are superior choices to anything else in the game.
That's true, but there is much, much more to the strategy than merely choosing what ships to build. Where and when should I attack? Should I prepare to get attacked? Should I invest in improving my economy? Should I do military research or would it be better to have more ships? Should I send my fleet to help my teammate and make myself vulnerable?
What I find ironic is that you're saying that there's more strategy to playing a dumb and unimaginative AI than there is to playing against human opponents online. The reason why your diverse fleet strategy works so well is precisely because the AI is dumb. (This isn't a knock on Sins; I've never seen a strategy game where the AI wasn't dumb, whether it's Civilization, Sins, Empire Deluxe, or Alpha Centauri; computers just can't simulate the strategic intelligence of a human.)
Online games just take too long to finish and its too much of a commitment to see one to the end.
They don't take as long as you think and most games are decided within an hour-and-a-half. Your average 3v3 game on the large random single star map normally doesn't last much longer than an hour-and-a-half. Is that amount of time so unreasonable?