Some doctors believe in conservative treatment period. They will only fix what they are sure is broken.
Some want discograms to confirm which disk levels are generating pain. An MRI may show many things, some or all of which though notable are not pain causative.

It would seem your MRI along with your current symptoms may be indicative of further pain generating disks. Without a diskogram...it is hard to know for sure. And after a fusion, the levels above and the level below the fusion may begin to degenerate... same thing happened with me.