PC#1 complete. PC#54 (still infected) re-infects PC#1. Even if you turn them all off and only put them back on the network after they are wiped: are you sure you found *every* PC with the infection? Are you sure they won't get reinfected again (is your baseline immune to the infection vector?)
Being completely naive and giving them the benefit of the doubt: someone suggested they planned a replacement cycle anyway (so had a budget approved) with w7 machines already - this expensive fix may have bought them a year more on one-foot-in-the-grave hardware. We're also assuming they had the staff/skill to do the work (unlikely) or to oversee the contractors and have the skill to verify the contractors did the job 100% correctly (unlikely). Plus everyone has to sign up to the risk of "we should spend heaps and if this happens again or something similar happens we all look like morons.."
W7 may not have been immune to the infection but given everything I've read in this thread, I'm leaning more towards speeding up replacement as the simplest approach here. But looks terrible as a news headline.