The V6 production ended in 2005 at Alfa Romeo Arese Plant, it was built a stock of five thousand, to be used Lancia Thesis, Alfa 166 and Alfa GT models. The engine was replaced in the 159 and Brera by a new 3.2 L V6 unit combining a General Motors-designed engine block with Alfa Romeo cylinder heads and induction. British automotive engineering company Cosworth was keen to buy assembly lines of Alfa Romeo V6 engine, but the Italian company did not want to sell it. The last version of 3.2L engine was Euro4 compliant, so it would been possible to produce it couple of years more. The engine designer Giuseppe Busso died only couple of days after the last engine was produced in Arese.
sounds like they built enough engines to keep rolling them out long after the production end date.