What he proposes makes a lot of sense if you accept that the purpose of prison is punishment. However, the more successful prison systems of the world focus on rehabilitation, not punishment.

Also, for incorrigible criminals, lengthy prison sentences do serve the purpose of keeping them off the street, while lashing does not.

Well, it's not just punishment lashing is good for. It's also very good as a discouragement of course. He does propose that for serious (violence-involving) crimes there should always be prison for the reasons you mention.

The most succesful prison systems of the world do not have to deal with millions of prisoners in organized gangs fortunately for them.

Singapore has caning. Perhaps someone did a study on that?

The best argument is that it's fucking barbaric, and if you need to consider that to reduce the prison population, your society is broken.

That's a closed minded argument.