Obadiah,
I'm not a theorist. I have done this full-time. I'm an ordained Christian minister and have pastored churches. I know what it is like to be a full time pastor and have to preach twice a Sunday and even lead a mid-week Bible study.
This is where I disagree with you. It is NOT hard to consistently come up with my own fresh original material for a sermon. I'm an expository preacher who sometimes deals with topical sermons (depending on the issues in my society, church and denomination). When I pastored a church in our capital city of Canberra, a local member of parliament was pushing for euthanasia to be legislated. I debated him in a public auditorium that attracted 500 people to the debate. It was necessary to become topical on that subject at that point. I've preached on abortion, baptism, illicit drugs, marriage & family, homosexuality, promiscuity, 'Conned by the condom', biblical authority, free will or determinism, etc.
I highly recommend Bryan Chapell's expository preaching text,
Christ-Centered Preaching (Baker 2005):
http://bakerpublishinggroup.com/books/christ-centered-preaching-2nd-edition/143610. It's the finest text and course I've ever taken on how to preach relevant, Christ-centered, expository sermons. And you'll NEVER run out of sermon material if you preach from Genesis to Revelation. I recommend alternating Old and New Testament books in a preaching roster.
In Christ,
Oz