To be honest, neither Shu nor Wu had a chance against Wei/Jin, after the north recovered from the battles at the end of the Han. Nowhere in Chinese history did anyone from the south rebound and conquer a united north. Wei/Jin just simply had too much land, population, and natural resources and if nothing else can wear out Shu and Wu by attrition (which is in effect what happened). Hindsight tells us that much.
Therefore, for either Shu or Wu to have a chance to conquer the north, some drastic measures are required. Either there had to be some big change in the northern regime that opens up opportunities for the two kingdoms, or the two must coordinate some very clever attacks and overwhelm Wei/Jin by good strategy. I think this much is indisputable.
Honestly, I don't think Wu ever had a chance of defeating Wei, and the only chance Shu had was Wei Yan's plan during the first campaign--the element of surprise was necessary. Of course, that was a risky move, and if Wei Yan failed all would have been shot to heck, but after that Shu really had no chance because Wei was ever on guard.
Of course, surviving a few years more wasn't necessarily going to do anything, but it certainly increases the chances of opportunities coming up. If Sima Zhao/Yan decided to overthrow the Wei throne early, it would have been possible to coordinate with dissendents and Wei loyalists and launch some kind of punitive action. Again, the success potential of this is unknown, but at least this is an opportunity. Dying early doesn't help you much.
I'll take advantage of Exar Kun's offer of "easy pasting":
Exar Kun wrote:The only they had had a chance was either to ensure they have talent of the highest quality all the time or to even the resource war.Once exceptional talent disappears it's certainly the end as greater numbers crush you.
I agree wholeheartedly. The exceptional talent has already disappeared in Shu. Jiang Wei is only one man, and none of the later generals are brilliant per se (compaired to Guan Yu etc). Wu wasn't chock-full of talents either, which was why Lu Kang kept petitioning to stay way from engaging in serious battle with Wei/Jin. Sun Hao ordered attack after attack and he was an idiot.
Regarding talents: You should really read up on the Shu bios, especially books 10 and 12. Unfortunately I don't have my SGZ with me over the summer, and I don't have much time to do translations either, so you'd have to ask your Shuists friends to help you with that one. I agree with LiuYuanTe and others above that Shu had fewer people than Wei or Wu, and statistically speaking, should have fewer talented people. Which is more the reason that Liu Bei and Zhuge Liang should have been on the lookout for talents. In the years Liu Bei was lord over Shu, the original Yizhou officers were mostly neglected. His policies alienated Liao Li, Peng Yang, and Liu Ba in the least (book 12), who were all known figures in the state. When they criticized the state, they were either exiled or executed. Fei Shi admonished against Liu Bei's claiming the imperial title and was demoted. Li Yan, who otherwise showed potential, was exiled. With a track record like that, it's little wonder that no one wanted to work for Shu, and all the Yizhou officials counselled Liu Shan to surrender! When you compare that to Sun Quan's forgiving Pan Zhang and Gan Ning, the difference between the HR policies in the two states is clear.
Also note how Jiang Wei had no designated heir. Zhuge Liang at least passed on his knowledge and aspirations to Jiang Wei.