Bill, first of all, I personally dont think the Taliban are stronger. I think their strength has always depended on Pakistani support (providing the skills they lacked and an international long term view) and a particular set of favorable circumstances and without that support, and in today's changed circumstances, they cannot conquer or hold afghanistan EVEN IF foriegn forces withdraw.
About why "many people" think the taliban is going to win (stronger was perhaps the wrong word), I think its as you said: people expect they will out-endure the US forces. The US has less of a permanent interest in Afghanistan and will eventually say 'f..k it" and leave after one last burst of bombing. The Jihadis in Pakistan (and they are the ones with international ambitions, the taliban themselves would be just a rural pakhtun phenomenon without their input) take a very long view of things and in their own opinion, they will always have more people willing to die than the US or any other infidel power. Once the US leaves, US agents like Zardari and the ANP will leave on the next plane (if they are lucky) and things will be back to status quo ante. I personally think they are wrong because they overestimate their own strength and unity and underestimate how much resistance there will be. Mostly, I think they overestimate their own unity. The army will not have the region back in their grip like they imagine and the jihadis will not have the army back in their grip like they used to. Instead, the US would leave behind an endless and extremely messy civil war in which Jihadi victory is by no means assured. Their extreme ruthlessness and clarity of purpose is not matched by any deep organizational unity. THEY will kill each other more efficiently than any infidel could (and the infidels will help). And their ideology has absolutely no section about how to be a modern state.....that part was a gift of the British raj and this time around it will depart with the infidels. But that is another story.
Some of the fence sitters have an idea about the mess that would result if the US leaves, but again, they are not sure the US can stick around, so mess or not, they have to place their bets on what will follow. On the other hand, if the US looks like it has a winning plan, then everyone else will start calculating differently. btw, "winning plan" does not mean plan to make a deal with the taliban and scoot. In that case, everyone knows who will be cutting heads next year in Kabul stadium and plans accordingly.