meadowrun
I'm pretty close to the action, having worked the 747 in general off and on for 30 years, and the -8 variant when it was still called the '747 Advanced'. I find the timing of this announcement quite interesting and more than a little curious.
Boeing has committed to producing only 6/year - which will stretch the current order book for about 4 more years. But it's not that simple - long lead parts need to be ordered 18-24 months in advance, so a decision to shutdown the line could come as soon as early 2019. Also, producing 747s at 6/year is not profitable - they need to get up to at least 1/month to make money on the deal.
OTOH, as you noted, the 747 Freighter provides capability that's not available elsewhere - if you want more than the ~100 tons of a 777F, the 747 is the only option. And 747-400s are getting pretty long in the tooth - 100k+ hour airframes are becoming the norm. Moreover, passenger operators haven't been parking their 747-400s until they've rung all the life out of them, so it's not very cost effective to convert them to freighters (and of course the conversions lack the nose loading door).
I wonder if Boeing isn't sending a signal to the freight operators - speak (order) soon or you'll lose the chance. When Boeing decided to launch the 747-8, they were very worried about the many open -400 production slots before we started building -8s. So Boeing offered the spots - at a good price - noting they would be the last 747-400s built. Scuttlebutt is that we had more interest than we had positions - we could have easily sold many more.