I think Smudger has hit the nail on the head. Whatever we may feel about Simpsons quality and ability at international level, you'd have to watch very little football to truly believe that Walker is as good as him.
I dont know what happened to Johnson at the World Cup, the first two matches was like 'Johnson Take on' with nobody else helping. From Slovenia onwards though he's fallen apart, caught out of position even more often, shocking mistakes and passes to opposition etc. He grew in to the Denmark game and I think he had a good 2nd half but you cant afford to be as bad as he was during the first 30mins at International level.
Point is, Simpson is no world beater, but if Johnson doesn't buck up soon I'd give him a go!