Well, your right about SP beeing important in the games, but for me there are three things that can keep the game alive for a long time
1) storyline and charachters - these are things that pull you in the game universe, an actually Wing Commander is the best example. without an interesting storyline the SP can get boring after quite a short time, even if the gameplay is real neat
2) editing posibilties - i'm pretty sure that makes a lot of people play the game much longer, if thay can make they're own missions and scenarios, or get stuff from other people from the net, as it keeps game alive after they have finished it
3) the posibility of comparing themselves with a reall oponent. Dont tell me that evryone will be replaying the gauntlets in standoff for so long if it wasn't for scoreboards
and MP a logical consequence - i mean killing more AI then somebody is fun, but actually shooting the guy down is more fun
I like the genaral idea, of the game where you can fly any ship, but for me it'll need either good MP or be easy modable and what would be best contain built in mission editor for the guys that are not that good with source codes