Life in prison without the possibility of parole for a financial crime does seem extremely harsh, no matter how much money was lost. I don't recall anyone getting that sentence after a big financial collapse. The bankers and mortgage agencies didn't even get punished at all back in 2008-09, and if SBF's lawyers are any good they'll get him off with much less prison time. Jesus, most murderers and rapists don't even get LWOP.
why is it cruel?
You do not know what situation the victims of fraudulent actions are in?
Moreover, the purpose of punishment - to show that committing such crimes - is very dangerous! And the punishment of one criminal should stop other potential criminals. When you understand that you will be in a cell for the rest of your life, and your whole life will be limited by the area of the prison, you will change your mind a million times over committing a crime. no, of course, someone will take a risk, in the hope that such a fate will not befall him ... But this is unlikely, there is a precedent legal system, and you will also have to sit in jail for life!