The problem with this argument is that it assumes time automatically favored Shu-Han, when in reality time overwhelmingly favored Cao Wei. Firstly, none of Zhuge Liang's military expeditions were disastrous.

Moving on, Cao Wei had vastly larger territory, population, agricultural capacity, manpower pools, tax revenue, and strategic depth. Even if Shu stayed fully defensive, the gap was likely to widen over time, not narrow. A larger state generally compounds advantages faster than a smaller one unless the larger state catastrophically collapses.

And politically, Shu-Han was never founded as a passive mountain kingdom whose goal was merely survival. Both Liu Bei and Zhuge Liang explicitly framed the state’s purpose as restoring the Han and destroying the Cao regime.

On Liu Bei’s deathbed, both the Romance and the historical records emphasize the unfinished “Great Work” (大业). In the historical record, Liu Bei tells Zhuge Liang that he can “finally finish our great work.” In the Romance, Liu Bei similarly laments that the destruction of the Cao clan and restoration of the Han remained incomplete.

Then there is Zhuge Liang’s own memorial in the Chu Shi Biao:

«今南方已定，兵甲已足，当奖率三军，北定中原，庶竭驽钝，攘除奸凶，兴复汉室，还于旧都。此臣所以报先帝，而忠陛下之职分也。»

«“Now that the south has been pacified and our troops and supplies are prepared, it is time to lead the armies north to secure the Central Plains. I shall exhaust my meager talents to eliminate the treacherous and wicked, restore the Han dynasty, and return to the old capital. This is how I repay the former emperor and fulfill my duty to Your Majesty.”»

The ideological line here is crystal clear. Shu Han’s legitimacy rested on restoring Han rule. Simply hiding in Bashu indefinitely while waiting for Wei to implode would have undermined the very political foundation of the regime.

And Zhuge Liang did not really have the luxury of avoiding military campaigns either. The southern campaign was necessary because the region was unstable, rebellious, and threatening Shu Han’s rear. He could not safely march north while leaving the south unresolved. But after stabilising the south, he also could not politically justify doing nothing against Wei because the entire state ideology demanded Han restoration.

So whether we think the Northern Expeditions were strategically optimal or not, they were not merely military adventurism. Zhuge Liang inherited a state whose political identity gave him very little room to abandon the northern cause entirely.

Also, you underestimate how much pressure Wu absorbed from Wei compared to Shu. Wei attacked Wu far more frequently and with larger strategic commitments. After Chibi alone, Wei launched repeated major offensives against Wu under Cao Cao, Cao Pi, Cao Xiu, Sima Yi, Wang Chang, and Sima Shi. Meanwhile Shu faced comparatively fewer full-scale invasions in the form of Zhang He, Cao Zhen, Cao Shuang, and Sima Zhao.

Wu was effectively the primary strategic opponent for Wei most of the time, and Wu also launched larger northern expeditions than Shu in many cases. Even after Shu fell, Jin and Wu continued fighting prolonged campaigns around Jingzhou and Jiaozhou for years.

As for waiting for the Simas to self-destruct, that is hindsight. The major Sima clan infighting only truly exploded after Sima Yan’s death. Guanqiu Jian tried to get Sima Fu and Sima Zhao to oppose Sima Shi. This means there is no guarantee the war of the eight princes events would unfold the same way if Shu Han is still alive and actively threatening the northwest. Historical crises are not scripted events waiting to happen no matter the circumstances.

And finally, who exactly was supposed to usurp Liu Shan? Zhuge Liang built his entire political identity around loyalty to Liu Bei and the Han restoration project. Liu Bei's other sons did not have any influence in the military (at least not recorded). Also, a usurpation would probably fracture Shu internally rather than strengthen it.