I think they would have gotten better. Their first game for the N64 was Killer Instinct Gold. Not exactly as well remembered as their later titles on it.

Being with Nintendo didn't stop Starfox Adventures sucking, so I see no reason why they'd get any better.

Sure, once you make a mediocre game, you can never recover......

There's more evidence to support a continued decline, (their negaive trend before leaving, their downward spiral afterwards) than to support a recovery.

Not really. Look at Silicon Knights. Didnt do well after Nintendo either. Rare were great at making Nintendo like games, MS never really gave them a chance to do that.

Rare's slump began before they left Nintendo though.

Theres no slump with one game. If their last 3 games with Nintendo got worse and worse, then maybe.

If their games suddenly turned bad after leaving Nintendo I'd believe that was the cause, but Adventure is just as bad as a lot of their Xbox flops, so I don't buy that Nintendo would have arrested their decline, since they not only failed to catch Adventures, but actively help ruin it by shoehorning in Starfox.

So if they made Banjo-Threeie for the GC, you think that would have gotten just as bad reviews and disliked as much as Grabbed By The Ghoulies?

Depends on how good/bad it was. The quality of Adventures and GBTG doesn't exactly fill me with confidence.