22 million is hard to fathom, so look at it another way. The Simply Sixteen's stitching speed is 1800 spm (stitches per minute) according to the HandiQuilter site. It isn't always used at top speed, so estimate that an average speed at which it was used is 1000 spm. (Totally a guesstimate.) That would mean it was used for 22,000 minutes, or 367 hours. That's about one hour a day for a year. Doesn't seem so bad now, does it, especially over the course of 12 years. I didn't realize the Simply Sixteens were even available in 2007.
You do need to test it out, and perhaps compare it to newer machines. Longarms have improved a lot in the last 12 years. Also check out the frame. Is it sturdy and easy to load? How old is the Prostitcher, and is it up to date? Lots of questions, but I wouldn't worry about the 22 million stitches.