The people mover is like an elevator going up and down. You can't really extend it very far, without making the vehicles so infrequent, it can't handle it. If you want a more sophisticated system with more than 2 vehicles, you will have to rebuild it from scratch. It's designed to have a very short extension to a second Terminal 1 station, if Terminal 1 is extended. That's it. Alternatively you maye be able to extend it at the other end to an adjacent parking garage. Anything would require a completely different system.
Replacing it to allow for extensions I would imagine would still be less expensive than building an Eglinton LRT extension from Renforth Gateway to Pearson though, as a revamped PM wouldn't require the complete reconstruction of the existing stations. Any LRT extension would require at least one pretty sizeable LRT station (probably elevated) built at Pearson, which wouldn't come cheap.
Like I said, as a standalone project it isn't very flashy, but when you start looking at it as an alternative to an LRT extension with a similar goal in mind, and it starts to make a lot of sense.