This of course assumes Steen is still playing for the Leafs in 10 years. And that Pronger would only be interested in playing for 4 years.
The thing I noticed last season, was that whoever was paired with Pronger, was easily our second best defensemen. You could probably put him on a pairing with Wade Belak, and suddenly Belak will be a good defensemen.
For us this season, Bergeron, Spacek, and Smith all played their best hockey when paired up with Pronger. He really does seem to be one of those "make your teammates better" type players.
I guess it always depends on what you feel the upside of the younger player is. And whether or not you think he can actually get to it.