In this case, the mage hero has spent 17 levels having "dedicated most of their pursuits to the perfection of" wielding magic in "single combat".
At level 17, it is legitimate for martial characters to participate in mythic battles with mythic powers.
For example, if the purpose of the Fighter class is to strictly represent what is humanly possible during historical battles, then that is a fine concept.
However, it is hard to conceive such a class concept ever reaching a level that is higher than level 8. Maybe level 12?
I consider the legendary descriptions of Beowulf to be pushing about level 12.
In other words, if this is the concept, there is no such thing a level 20 Fighter.
The Fighter would function better as a Prestige class that one can multiclass into, that only has levels 1 to 8 (or 12).
Or, in the context of Pathfinder 2, such a "realistic" Fighter would only be feats that are available from levels 1 to 12.
But, if the purpose of the Fighter class is to reach level 20, then at some point the designer must make sense of reaching upper levels that are clearly superhuman compared to a "realistic" soldier.
The level 17 Fighter must be able to do the equivalent of a Wish spell − every day.
Supposing the Wizard warrior mage has a slot-9 spell at level 17.
The Wizard player can choose whether to spend this slot to cast a Wish spell, or else spend this slot to perform a legendary magical event during single combat.
In terms of design space, there is opportunity cost, and the choice of how to spend the design space maintains overall gaming balance.
One can bring up balance concerns about versatility, being able to cast different spells. Counter concerns include.
• The benefits of versatility are often exaggerated and nerf-hammered.
• In P2, the continuation is of vancian prep, more painful than versatility is good.
• The Fighter design refusing to do things beyond level 8 (or 12), not my problem.
A mage hero that can defeat opponents can balance.
There is nothing magical about a "sword" (pun intended) that makes a sword necessary for the concept of winning single combat.
Even if this particular warrior mage chooses to wield a sword, this sword might be dealing force damage and be clearly magical.
I dont want the martial dabbling of the Magus. I want the mage to be the hero. Not the half-asked martial to be the hero.
The mage who wins fights.