Success can be a hard thing to quantify. I admit, I thought Lennon would do better at SIU than he has, basically because of his prolific tenure here. However one probably has to consider the heirarchy of DII and DI when measuring his success.
For instance, did you know that Lennon has the 2nd highest winning percentage at SIU, behind only Jerry Kill? It is not like SIU is to DI what North Dakota was to DII, which is/was a perennial top program. In 95 years of football they are 50 games under .500. North Dakota is 250 games over .500. SIU has had just 6 head coaches with winning records. North Dakota has had 15.
My point is, it is probably easier for a good head coach to have success at North Dakota than it is at SIU, for whatever reason. Money, institutional support, regional competition, etc. I'm sure the SIU faithful and Lennon himself would have preferred to make the playoffs the last couple years, but I'm not sure what they realisticaly expect (the fanbase that is). It is not like SIU is a Montana that has been a favorite to win a conference title for like 25 straight years. He's already the 3rd most successful coach they've ever had, and his teams have improved each of the last two years.
Jut my 2 cents.......