The 340-600 is a new addition to the fleet! Virgin has 7 now out of 12 firm orders, with probably more to come. There is no chance of the 346 being replaced by 777/744. The 343 will be the next fleet to retire, and the older 744s will be next (Virgin's newest 744s aren't yet a year old). The 777 or further 346 orders are for expansion, not replacement of anything, anyway.
And the 346 burns as much as a 744? Less freight? Do me a favour - go and check your facts, capt2ezy, you're way off!