The inspector may not " require" the wiring to be replaced but it not as cut and dried as it sounds.
You have a fire like this, the power company should remove the power meter and there is no electrical service. The house is considered un-inhabitable ( condemned ). Now it becomes a matter of what will be needed to get a new certificate of occupancy. The inspector may not require that the wiring be up-grades but they will have to find a contractor that is willing to take responsibility for saying that he/she has checked out the electrical system and it is safe. There are not many that I have seen that I would be willing to say that they are safe. Between the fire department and the demolition crew that removed the drywall there is normally at least some damage. You add this damage to all the modification that have been done over the years and it starts to getting cheaper just to replace existing wiring rather than try to make repairs and warranty the job.
In many ways the inspectors let the contractors decide how much risk they are willing to take. Most of us wouldn't want any part of leaving that old wiring because that's taking responsibility for something that's not being paid for.
