Yes, the Diplomat was the last RWD mass produced car until now with the LX cars. Of course the Viper doesn't count since it isn't mass produced.

The truth about GM is they refuse to give up on anything. Look at the W-body, the Cavalier, or the SUVs they refuse to give up on when they are obviously not popular. GM doesn't like to be innovative, they prefer to hold on to stuff forever, sometimes it pays off like the Corvette, other times it doesn't (too many to list).