[throw310822]

The objection that perfecting something beyond what's practical would be unfeasible is not a good one against making it better within reasonable limits, right?

It's very difficult to quantify the harm that jail does to you. It depends on a lot of factors, some of which are entirely personal, and besides, jail has multiple functions (in short: dissuasion, retribution, prevention and re-education). A fine exists mostly for dissuasion, and has a very precise economical value that is easy to gauge on the income/ wealth of the person fined.