I think as long as it is 4th level and below it is ok. I find money and time to be significant barriers to copying spells and you are usually limited on how many you can prepare anyway.A discussion on how powerful the wizards “breath of knowledge” is.
let us assume a common scenario in some campaigns.
The party’s wizard is an alumni at a Wizards academy. An alumni perk, access to their spell library. while the wizard must supply their own inks, they effectively can have any core spell in the game if they use their downtime to swing by the alma mater.
compared to the standard wizard who finds the occasional scroll for copy, how “strong” is this?
how much better is the alumni wizard in comparison? Or put another way, how much money is the privilege or all of that spell access “worth” should the school charge its alumni for access?
For this purpose let’s assume the wizard is your typical adventuring PC, receiving normal treasure, and gets some downtime between each major adventure.
Once you get to 5th level I would give them a chance to have a specific spell on file in the library, decreasing with every level through 8th. Maybe 80% at 5th level 20% at 8th level.
9th level spells would be special, while the head of the academy might have some in his personal collection, but they would not be in the library. Of course said head of the academy might be willing to let an aspiring young 17th level wizard peruse said collection in exchange for a specific service or quest.