The Hemi engine's downfall is its inability to adapt to features proven to increase efficiency. The large combustion chamber design and dual valve structure leads to blow-by of unburned fuel, thus producing large levels of undesired emissions. The use of dual spark plugs has improved burn in the hemispherical combustion chamber, however, it's a solution that increased cost and complexity. More modern and efficient chamber structures, unlike the Hemi, can fit four valves and utilize dual overhead camshafts. Chrysler plans to implement these features into their future V6 engines, along with cylinder deactivation and variable valve timing.
[Source: Automotive News - Sub. Req.]