My view on Metrojet vs. Delta Express vs. "Song" is that there are/were several vital differences between US's low-cost carrier-within-a-carrier and Delta's.
I believe one of the key differences relates to the hub structure of the two airlines. Delta has ATL, US has CLT. Before 9/11, ATL was operating at or near capacity. Delta was, in part, using capacity at its smaller hubs like DFW to reduce some of the load on ATL. DL Express flights operating point-to-point also reduced traffic flow through ATL, freeing up seats for higher-margin connecting traffic or O&D traffic through that hub. Compare that with CLT. US Airways' O&D traffic at CLT was 21% of the size of DL's O&D traffic at ATL in 2001. In order for the hub to be viable and to offer a schedule with frequent departures, US Airways can't afford to siphon much traffic away from CLT. To put that into perspective, demand from CLT to/from BOS in Q2 2002 was roughly 175 passengers each way -- enough to justify roughly two daily A319's just for O&D (compare this to 8 daily round-trips on the route). Demand between BOS and ATL was nearly 1,040 passengers each way per day. When US Airways was overflying CLT with point-to-point Metrojet flights, the flights into the hub were drawing fewer passengers, making them less profitable.
Another key flaw in Metrojet was strategic. IMHO, it was initially viewed as a loss leader to slow Southwest's East Coast expansion and to protect market share in the Washington/Baltimore area. I believe that was a grave miscalculation, given Southwest's dramatically lower cost structure and what was clearly a long-term plan to build a major operation at BWI. WN was in a position to make a profit even while directly competing against a money-losing Metrojet, and would have been able to do so indefinitely. And, moreover, US ended up competing against its own mainline operation, both for connecting traffic and by causing spill from PHL to BWI.
Metrojet also lacked a consistent strategy. Management bounced it in and out of routes with flights to places like MKE, SYR, etc. -- in some cases, routes with little to no low-fare competition.
As for the 757's -- today is a bit different than three to four years ago. At the time, US actually didn't have many aircraft aside from the 757's which were suitable for the higher-density long hauls to the West Coast; the addition of the A320's/IGW A321's has helped in that respect. And the 767's were needed for the transatlantic routes, of course.