Sprint makes money selling phones. Sprint got your money when you bought the Evo 4G. Sprint must continue to release new phones with features that the older phones don't have, otherwise nobody would have an incentive to upgrade and Sprint wouldn't make as much money.
Will Sprint release gingerbread with Sense 3.0 for the Evo 4G? I don't know. I can only say that it would result in more sales if Sprint didn't release Gingerbread with Sense 3.0 for the Evo 4G and instead released Gingerbread with Sense 3.0 for the Evo 3D.
Will the Evo 4G get Gingerbread? I don't know. I can imagine that, if the Evo 4G did get Gingerbread, it would be sometime after the Evo 3D is released to give customers an incentive to upgrade to the Evo 3D.
Just my $0.02