Optional vs. Mandatory
Mandatory Service Bulletins are paid by the manufacturer: Thales or Airbus in this case. Optional SB are paid by the user, and on the user's schedule. The maker tends to downplay the urgency, in order to avoid paying the cost. It takes a diligent regulator to correctly accept the maker's determination of mandatory vs. optional.
If an airline wants to incorporate an SB Free of Charge, it often has to argue the case. Pending orders for new planes affect these negotiations, of course.
Assuming there was a limited supply of new pitot probes, it would have been prudent to do a phased upgrade, doing one at a time on each plane until the full fleet has one new pitot. Doing all 3 at once on each plane was a mistake, not only for logistics, but also if something went wrong with the upgrade.
If just one pitot on AF447 had been replaced earlier...