Hubble can see things that JWST cannot and can be aimed at other features besides what JWST is tasked with.
The main problems are orbit and loss of the stabilizing gyros. It makes no sense to boost it higher if the gyros aren't replaced. For other satellites a general proposal has been made to add a separate guidance back-pack that would include orbit keeping and stabilization without having any modifications or replacements made of satellite hardware. On Hubble this would be attached to the same fitting the Shuttle arm used. In a more ideal world, the Space Shuttle would retrieve Hubble, return it to Earth for an overhaul; maybe fix the primary mirror properly, and then send it back into orbit. That appears to be no longer possible for the next many decades.
There may also be a science funding issue that continued operation of Hubble prevents the funding of a far more capable near-Earth telescope.