Arcana, the skill, is knowledge of magic and how it works.(Bolding mine)
How is Arcana not in a wizard's niche? If you have two characters who specialize in arcane magic... they're going to get in each others niches....
Or am I missing something?
A Rogue can become an academic, like Dr Jones, who knows everything about how magic works, but can't do any of it.
They might know more magic theory than the Wizard, but they cannot cast a single spell.
The Wizard is the applied magician.
As an applied magician they have picked up a pile of magical theory. But not as much as Dr Jones over there.