People are smart enough these days to know, who's the market leader in certain categories. The walled garden approach will not deter them from seeking out the best product just because Yahoo!'s own product is featured on their portal. This is essentially an early version Internet strategy, or the AOL approach, which will work for novice computer users, but as the population becomes more tech-savvy, will show ever diminishing results.
One strategy that IMHO makes sense to me surviving as a portal (aside from their web properties), is that of a personal aggregator, like netvibes, so the front page becomes a customized dashboard reflecting users interests.