Time for a new plan?

Does this make sense anymore:

But Mr. Kerry also made clear that there would be limits on Iran’s role if Tehran did not formally accept that the goal of the conference would be to work out arrangements for a transitional authority that would govern Syria if President Bashar al-Assad could be persuaded to give up power.

Why are Western powers still insisting that the solution to the Syria crisis must involve Assad leaving power? I'm no fan of Assad. And it wasn't all that long ago that I also assumed that Assad was basically a goner. But a lot has changed in the last year. If you want to resolve the crisis as it exists now, you need to be realistic about what is likely to happen. Assad's grip on power is no longer tenuous, at least not in the portions of the country that he never lost. If anything, his side is the one with the wind behind its sails. There's no reason for him to agree to step down anymore. And also, given how anti-Western militants have increasingly dominated the anti-Assad rebel groups, it's not clear that Assad departure would be better.

So why are they sticking with last year's plan? There is no reason for the U.S. to get sucked into the Saudi's anti-Shia madness.