I was just thinking and realized something not sure if it will play a role or not, But We know zetsu can absorb and give chakara, he gave sasuke chakara during the kage summit , we also see these first version of zetsu are open and meant to be worn as a suit ,
The only conclusion i can come up with is this was done on purpose by madara and he is simply bull shitting to obito saying he cant leave , in fact he can leave because he can receive the chakara from the zetsu , the zetsu are grown from the same tree he is linked up too,
because i ran across a problem with my original theory , i was thinking that obito started the bloody mist tradition out of revenge for rins death, i.e making the children kill each other to become a gennin, but we know that this tradition stopped when zabuza come along like a boss and killed over a hundred fellow students, zabuza is about the same age as kakashi , and he became a genin at age 9, so obito couldn't of really started it , so its my assumption that madara was the one controlling the 4th mizukage wearing a zetsu suit like a boss ? ?
madara obv designed the zetsu to be worn like a suit on purpose