I've lurked and learned from PPRuNe for several years; this is my first contribution.
There *was* a 747 with a purpose-built crew escape system, specifically NASA's Shuttle Carrier Aircraft. It was installed during the "drop" test when the orbiter Enterprise was released off the back of the 747 over Edwards AFB.
The crew wore parachutes during the rather risky test flights. If they started an evac sequence, about 30 cabin windows along the fuselage would be blown out by squibs to cause rapid but controlled depressurization. A hatch about 30 feet behind the nose gear would be jetissoned after the pressure equalized. The route from the flight deck to the hatch was a slide/tunnel similar to the ones in kid's playgrounds.
I had a very keen interest in aviation even as a kid. My friend's dad was a bigwig on the Shuttle program and thanks to him I got a tour of the inside of the SCA and was a semi-VIP guest at 3 of the glide tests. An engineer on the SCA was nice enough to answer my zillion kid's questions, including the escape system.
About 10 years ago the SCA was on static display at one of the last Edwards airshows. I noticed the escape system was removed. The pilot said it predated him, but he knew a lot about it. He heard it always made everyone very nervous so it was disabled as soon as the test flights were finished. He also said that had anyone actually used the system they probably would have gone into #2 or #3 according to later CAD / CFD analysis.
Pointing out an actual example of 2 points already made.....Parachuting out of a transport jet is tricky and risky even with a purpose-built system. And, the safety system can impose a new set of risks.
BTW, the original SCA is a converted AA bird; the second is a JAL 747SR. Altitude is limited to FL240 when the shuttle is on top due to cold temp limits on some shuttle systems. That's why the ferry flights require a mid-continent fuel stop.
Most important of course is the UPS crew and those they left behind. The cause of the accident will hopefully help advance safety, especially for inflight fires.