You should absolutely be generous and let PCs be powerful at high levels, but there's a big difference between that and letting them get away with anything and everything.
I mean, why stop at an extra 9th level slot? Why not let the wizard wish to have infinite spell slots? Or let them wish to become the most powerful being in the multiverse?
Wish is described as the most powerful MORTAL magic. Meaning that its power is not unlimited (if it weren't, why have other 9th level spells, or even other player characters). There are plenty of examples of what it can do in the description, which are plenty powerful. I see nothing wrong, however, with saying that there are things beyond the scope of a mortal wish, or within scope but risky and unreliable due to forcing the wish to the absolute extremes it is capable of.
Wish is normally intended to be a plot device rather imho, even if the wish is worded to gain a powerful item. A wish should be viewed at most benevolent the less personal power gain is involved with it.
E.g. the party cleric died, no one can cast a resurrection, so the wizard casts a wish for that purpose.
A powerful weapon is needed for a unque opponent who can only be overcome with e.g a +3 weapon of disruption (think demon lord or whatever), although it makes the character wielding it quite powerful the purpose in this example justifies the means.
Abuse otoh should be punished by the DM in a humorous way.
E.g. The party mage is risking his 33% chance to become the party outfitter, he starts with wishing for a +3 plate armor, next a +3 shield , a +3 ring of protection, a +3 cloak of protection etc etc.
He is just one of these people with lucky dice (they exist, blime). At some point the wish should produce e.g. a shield and 3 other shields instead (unless the DM is fine with that kind of gaming)
A wish like the OP stated, for a permanent additional spellslot of the highest level is a clear abuse, and I think the intention of the PC wishing for that slot was to get it permanently. This is working around checks and balances of 5e, if it were not like that the number of spells of 6th or higher would not be limited to 1 no matter what.
As some above pointed out this could be an epic boon for a level 21+ player as a reward by a god or so, but never just by casting a wish.