The author never suggested that thermo. generators would be the most cost effective means of using the currently wasted heat. Even if there were more cost effective means, the thermo. generators could still be one way to save money.

The conclusion is based on the hypothetical situation in which the plants could feed heat into thermo. generators. If that’s possible, the author says, then it would save money. But this doesn’t require an assumption that the hypothetical situation is in fact true with current technology. Even if current tech. can’t do it, we can still argue about what would happen IF it were possible.

Necessary, because if it were not true — if the savings on electric bills would NOT be enough to cover the cost of purchasing/installing the generators — then the generators would not result in “saving money.” The savings from electric bills would be offset by the other costs.

Not necessary, because as long as steel plants use electricity at all, the author’s argument can still work. There can still be savings on electric bills, even if electricity is not the primary source of energy for any steel plants.

Although the author assumes that some plants could reduce their electric bills by converting heat/energy into electricity, this doesn’t imply that some plants need to use such conversion in order to reduce their electric bills. Even if the conversion isn’t required to reduce electric bills, it can still be a method that would lead to a reduction in electric bills.