Absent any other arrangements, a poor person signing a prenup is agreeing to remain poor even after the death of the wealthy partner.
This is not necessarily the case. If the wealthy partner wants to reduce the chance of the prenup being contested, he/she would be wise to agree to reasonable, even generous terms. I'd suspect many prenups are written this way. Unless contested, prenups also of course potentially save legal fees in the event of divorce, which can be enormous.