It has not gone unnoticed that if you can boost it down, you can boost it up. The question is how much of the telescope is still working when you decide to do it. For example, the NICMOS infrared instrument is not currently working because of a problem with the cooling loop. The other instruments are all working well, so the telescope may have quite a long life span ahead it it, even without repairs.
The cost of continued operation is small -- on the order of a few 10's of millions of dollars a year, so after all the initial investment, you might as well get as much as you can out of it.
If SpaceX finds a way to refurbish Hubble inexpensively some day, that is all the better.
Biting the hand that feeds IT
