At 18th, the wizard has only 1 slot each of 6-9th level, yes these should be a valuable resource and the DM needs to make sure they're not constantly replenished/available.
Also by that level, lower level slots are somewhat spammable sure, but the wizard needs to be pressed with ways to use them up - counterspell, shield, absorb elements, etc. If the wizard is only using slots for the stuff they want to use slots for - they are not being pressed enough. And if the wizard is constantly allowed to replenish (short or even long rest whenever they want), pressing them is nearly impossible.