McNabb has to be one of the most insecure elite pro athletes of all-time and I'd agree that it's pretty clear he feels threatened by Wentz based on these comments. What is the significance of making it to an NFC championship? Very little for most I'd say but for McNabb it means something since he made it to and lost something like 4 or 5 NFC title games in a row. Had they not lost that many in a row nobody would even know who lost those games. Do people marvel at Belichick and Brady for making 8 straight AFC title games? Nope, but they so for 6 Super Bowls wins.
If McNabb wasn't so insecure about his own legacy in Philly he'd state the obvious and say Wentz needs to lead them to another Super Bowl win to prove he's worth the money and notoriety of being a "franchise QB". He won't though because he would defame his own legacy then.