Whilst I've got no gripes against the Legacy Cast (hell, I can barely remember their performances), it's not like they're iconic. There wasn't a great deal of FFVII media with the main cast being fully voiced (Advent Children, Crisis Core and Dirge of Cerberus is all I can remember off the top of my head). Also, as everyone above has basically said, the Remake cast knocked it out of the park and don't see a reason for them to record an alternate audio track. The fact that they made it clear that the Remake throws the Compilation of FFVII out of the window is reason enough to not use the old voices, imo.