If I were a junior Capt on the 777, I'd rather be a on a junior fleet now with the knowledge that one day when I have advanced in seniority enough, I'd be able to move to the fleet of choice. As it stands now, the entire system is a one-way crapshoot. The optimum fleet today will most certainly not be the optimum fleet a decade from now.
Of course there is a tangible cost to this, with intangible benefits, so it's safe to say it'll never happen with this management style.