How long can these channels stay open?
Indefinitely.
What happens if I send a tx to C and B closed his/her channel?
Then the transaction fails. C does not get his money from B and B does not get money from A.
Will the LN still generate enough tx's to sustain miners fees for miners to continue mining or will they have to switch to running LN hubs to complement their income?
No one knows; that requires predicting the future.