Not a fan of overstretching the military, particularly when our plan was dependent on drawing down forces at calm borders (ex. the one we had had with France that is no longer so peaceful) and we have a certain ambitious guy's goons in Sichuan where we'd have to hollow our forces out ones so we can have them fighting further south.
Like I get that as far as France is concerned they're themselves stretched so one-on-one we could probably easily take their glorified police force while they're busy anyway with the existing dissent and commitments way out in Europe and the Americas. I absolutely believe that part of the text.
But what could happen if say France (which has preexisting and recently friendlier turning communications with former Taiping officials) offers support and promises for those disaffected elements to launch a revolt from behind our lines? We'd have a lot of juicy supplies moving thru the Sichuan basin and only token forces to guard them and keep up the reconstruction effort while we support a raging front south of the mountains. If timed correctly an uprising could overwhelm the local forces in the basin, secure the traitors a mountain-ringed redoubt, and chop off the supply lifeline to the southern forces leaving them to wither while sandwiched between rebels and invaders. We could stand to lose much of what we gained in the West China War, never mind add something new. We are also well aware with how unfortunately this sort of hypothetical act by the French lines up with the known blind spots in our intelligence so we could be caught completely off guard.
Just wanting to caution that waging war with only an eye for how to maintain our majority can lead us blind to greater risks. If new territory brings us more political forces to incorporate then as far as I'm concerned learning how to handle coalition building/navigating a coup amidst our government squabbling in a bigger China is better than how far south things can go if our militarily overstretched state is capitalized on in the sending soldiers option.