Still cheaper to replace than service
A whole new launch on expendable vehicles* is cheaper than a shuttle visit to repair the problem and also puts 100% brand-new insfrastructure up there, with every part brand spanking new.
Servicing only fixes the few broken parts they designed the mission round. Not anything else that's getting elderly, close to failure, or gets damaged during the repair itself...
* Assuming that Uncle Sam has anything left in the cupboard that can lift a newer smaller Hubble. Maybe he should ask the Russians for some help?