I honestly agree with this. His run on Def Jam has been nothing short of amazing, however I think that owes more to Nas being reinvigorated than to Def Jam as a label, they haven't really marketed him that well outside of Life Is Good(his best label based promotion in YEARS) but the opportunity to work with Jay-Z, No ID, Kanye West, etc obviously motivated him and sparked a creative tear that began on Hip Hop Is Dead and culminated with the masterful Life Is Good. Hip Hop Is Dead is an excellent album, Untitled is a classic to me, Distant Relatives is near classic (though it wasn't on Def Jam) and Life Is Good is a masterpiece. The one misstep that they made was not allowing him to release Lost Tapes 2... He however has gotten 3 STRAIGHT number 1 albums, 3 Grammy nominations, and an Emmy win out of the deal
I DO NOT agree that Nas was headed to irrelevancy while on Columbia though, releasing 6 straight multiplatinum albums, a Gold selling QB affiliate album, and a compilation of unreleased material that debuted in the top 10 and sold 340k is NOT the definition of an artist headed towards irrelevancy

Stillmatic and Gods Son are amongst the most universally acclaimed albums in his discography and Streets Disciple sold well and also was acclaimed. Nas has NEVER been the type of in-your-face artist whose on every radio station/awards ceremony/tv show promoting himself. Life Is Good has been the first album in YEARS in which he's actively promoted himself through these outlets.
Honestly, if he decides to stay with a Major, I don't see why he wouldn't resign with them. He can work with the masterful No ID, has access to Kanye West and any other producer he wants to work with. It seems like their high on him as the current president said that their knee deep in "Nas business" during his prerelease session.