He never was. After Fall of Reach it was ambiguous at best, Linda was thought to be dead but in 2003 Halo: First Strike revealed her and the rest of Blue Team to be alive. In 2006 their story continued in Halo: Ghosts of Onyx, Linda, Kelly, Fred and a handful of Spartan-III's were trapped inside a Forerunner shield world while Chief arrived and fought on Earth. In 2008 Halo: The Cole Protocol introduced Gray Team, whose fate was left ambiguous and their adventures continued in Halo: Envoy, which funnily enough just came out yesterday. In 2009 Halo Wars and Halo: Evolutions introduced Red Team and Black Team, Red Team entering cryo at the end of Halo Wars and never being seen again until last year's Halo: Fractures and this year's Halo Wars 2. Black Team died just recently in one of the most fucking atrocious pieces of writing 343 has made. In 2010, Halo: Reach left Jun's fate unknown, which 343 followed up on by making him one of the heads of the Spartan-IV program. Chief was never the last Spartan - not even close.

And no, Revglenn didn't. He said both Osiris and Blue Team were Spartan-II's and how Chief was no longer the last of them.