Noone's talking about extending Hubble's life expectancy because if we admit we can extend the telescope's mission, people will start asking if we really need to spend a new fortune on NGST while a perfectly good Hubble is on orbit. Partly because of all of the lovely construction contracts which can be spread around when the NGST mission is approved/partly because NASA doesn't want to be seen to be unable to get its act together on another big international project in the midst of the ISS fiasco, the necessary evil is that Hubble will be summarily retired the minute NGST goes up, even though there's no good scientific or technological issues which require it.

Robert