electron transport chain complex II question

how come we are still able to produce atp if complex II was to get mutated. I did a question yesterday and it was asking what would happen if complex II stopped working, and the answer was that the degree of atp production would decrease. I thought we need complex II to be able to transport the electrons which will help the complexes proceeding complex II do their job.

and so far every question regarding a complex of the etc being mutated the answer seems to fall under the fact that atp will still be produced just not as much. I was wondering why is that and when do we see a complete stop of atp production?

Edit: I’m asking regarding to the ATP that is generated from oxidative phosphorylation and not other processes.