Matt Cassell is not a top 5 qp; Matt Cassell will not be franchised; No team would be willing to pay Matt Cassell to be the 3rd highest paid QB in the league and also be willing to give up talent/draft-picks as well. Matt Cassell if he had the chance to be the 3rd highest paid QB would not turn it down for a long term contract making him the 15 highest paid QB (something like 4 years 20 million with less tha 10 guaranteed would be reasonable). Matt Cassell won't even have a guaranteed starting job next year; he'll just have a "You're the lead but can easily lose it" job.
Cassell is playing well, but he's still got the same offense around him that last year had the best offensive season in history, and that kinda helps his stats. Cassell will not play as well next year with lesser talent; he's not the missing piece to bring a 2-14 team into playoff contention.