"I believe that most criminals should not be sent to prison as to do so is just a waste of time and money."
Substitute the word 'some' for 'most' and you might find me agreeing with you. Your statement that "Most just re-offend on release anyway" is technically correct, although the re-offending rate varies in different prisons. What isn't correct is your inference that if we didn't send people to prison they wouldn't re-offend at the same rate. There's no evidence to support that theory.
It's a highly complex and contentious subject, but on the whole I'm in favour of imprisoning people for violent crime, serious drug offences, serious fraud, murder, rape, and cruelty to children. Other convicted criminals could be set to work in the community.