Solomon and Johnson aren't young guys in the world of 800 meter running. They're both 28 years old, a year younger than Symmonds. Both competed for years (Johnson in the hurdles until last year) previously with much less success than they're having now. They actually belong to the same era as Symmonds.
To date, none of the actual young guys, like Loxsom, Greer, or Sowinski, have beaten Symmonds in the 800. So far there's no reason for him to move up to the 1500 unless he wants to. His new 1500 PR does indicate that he can if he does want to, and it indicates good strength for getting through the rounds in the 800 at Moscow.