They don't have to. Tou can stay in Han to defend it and places like Nanyou and Shintei can keep their defences up like they have.
You think Tou could defend Han against a Coalition?
Qin is rebuilding his army. I believe that Qin conquering Han will be a big shock in the manga and that the forces that Wei and Zhao sent will be stopped by Ouhon and Mouten.
Tou will stay in Han with his army to defend Han and it will be enough.
Shin, Mouten and Ouhon will have bigger roles further up the road. Ousen will also make a comeback with a stronger army than before.
After that, the other states will start to play solo imo and prepared for a Qin invasion.
Do you not think Ou Hon and Mou Ten would be recalled in the event of a Coalition forming?
This is purely hypothetical of course, but in this instance it would Chu getting involved as well, so I imagine the Qin would do what worked for them the last time: fall back and consolidate troops at location favourable to defense.
As it stands, if there was another Coalition to take back Han, I believe the Qin would be in a really difficult position Ou Sen and Mou Bu in particular. Maybe YTW would be spared from the Zhao fronts, but I would think Ou Sen needs her back now more than ever while he trains up his new 80K recruits.