I imagine WotC is putting their trust into two things here...What I don't get is that if the Essential's Fighter powers cannot be used alongside the PHB1 Fighter...why use the same name?
If they are effectively different classes that accomplish the same goals, they should just use a different name to prevent confusion. Fighter in PHB1 = Soldier in Essentials for example.
1) That established, current, 4E players are actually intelligent people who will be able to recognize the similarities and differences between the PHI Fighter and the Essentials Fighter, and be able to work with them both without getting their brains tied into knots.
2) That long-time, former, early '80s players of oldy-time Red Box Dungeons & Dragons might pick up the new Essentials Red Box on a whim or in a fit of nostalgia, open it up, and find it to have practically the exact same terminology they remember from their youth. Things like 'Human', 'Fighter', 'Dwarf', 'Cleric', 'Halfling', 'Wizard' and the like. They then recall the good times they had as kids, continue to read the info inside the box, and then maybe, just maybe, show the box to their sons and daughters and say 'You know, I played when I was a kid... what say we turn off the Wii and give this a try tonight?'
Changing the 'Fighter' to 'Warrior', 'Cleric' to 'Priest', 'Wizard' to 'Mage', and 'Rogue' to 'Swashbuckler' does not really gain you anything, other than four "new" classes to add to our current list of what, 24?