I don't know what junction temperature that 50% degradation chart is calculated at, but assuming you don't drive the lamp at 100% all of the time that it's on and it's properly cooled, I would suspect you'd go a lot longer than 50k hours before output is down to 50% of new.
What would be...
DKChris, for a 72" (like mine) I'd go with 1x BT-EX 120W master and 2x BT-EX 120W slave, or a 60" 140W Diemos (like I have ordered).
Pictures of the Black Python units in the first post of this thread here...
An interesting counterpoint to your view on red can be found in this AA article http://www.advancedaquarist.com/2008/12/aafeature1...
That said, the internet is full of opinions, and this is just one of them.