1) The electric propulsion will take months! Months without any observations, no science, as it will have to be aimed towards the station.

2) The ISS is in a much lower orbit and the HST should be pushed again to a higher orbit, after its refurbishment from ISS, which will also take time.

3) ISS has a pretty polluted environment, lots of atmosphere (leaks form the pressurized modules) and lots of loose pieces (we recently heard of 2 in a week) around it.

Solution:
As long as HST works great, produce as much science as possible, now, until too many gyros malfunction.

I really don't understand that obligation to control its re-entry: the Solar Max, the GRO-Compton, etc...have re-entered earth's atmosphere and no damage resulted from them.
The HST orbits near the Capricorn and Cancer tropics, not further from the equator. Many fire insurance policies cover such damage.
And if such a piece would fall on someone's property, the chances are much higher he'd either be able to sell it to a collector (and largely recover from the eventual damages) or keep it as a priceless heavenly treasure (this if he decided not to tell anybody)!

Raoul