A couple points, just to.play devils advocate.
X has had about a season and a half of "elite" play out of five, so to assume several more seasons at that level is not only speculative, but given the fact, historically, great corners tend to fall off in terms of performance in their late 20s, is also unlikely. Couple this with his well known injury history, and it seems to me that long term gaurantees are an unwise, and risky proposition.
Also to say our chances of winning a SB are so much better, while true in a theoretical vacuum, are far more nebulous in reality. It completely dismisses development of younger players, cap implications in regards to paying other players and FAs, etc.