every other video game genre works on a two or three year cycle and seems to be doing OK.
with some creative moves, good DLC content and selling of updated rosters I bet the game companies could make up the difference in revenue they lose.
for instance one thing they could do is stagger releases. not everybody buys both NCAA and Madden each year. but if Madden releases in 2008 and NCAA in 2009, folks might buy both that normally wouldn't.
Same for NHL and NBA Live. or NHL 2k and NBA 2k.
not to mention sales are very likely to increase with a superior product.