Our old software was extremely out of date. The developer sold it off and the new owner retired it years ago. We were painted into a corner, unable to upgrade our server hardware or software and patch known security holes, because the old software wouldn't work on new server software. We limped it along for several years after the writing was on the wall.
To put it bluntly, we were a sitting duck website. One hack or hard drive failure and we would have had to perform this upgrade in an emergency manner, probably lose a lot of data (think about having 550,000 permanently broken images) and have several days of downtime. Since our database has been migrated several times already, the artifacts left behind from those past migrations became problems, which took the better part of a year for us to sort out and finally get a reliable path to this latest migration. Otherwise it would have been upgraded long ago.
Another big issue was that we basically had to stop all development of new features due to the old software being abandoned. So that meant other parts of our website started to fall apart, like the articles, classifieds, and reviews, because they were all custom built and tied into the old software. Now we can regroup and begin developing new features again.
I think once everybody gets used to it they will find that we didn't lose any features but we gained a bunch that will be very useful.