DOJ may just want to in some way encourage the court not to make a "mistake" in terms of the length of the sentence, which is a good move considering what the defendant's lawyers are trying to do. In my opinion, the appropriate punishment is the one that would keep Bankman in prison long enough to prevent him from repeating something similar, but also to send a clear message to everyone who has similar intentions.
A higher sentence also means that Bankman will have to be in prison longer before he can ask for a pardon, so even 40 years doesn't seem bad considering how much he could actually get, which is 100+ years.