But then aren't there other obligations as well? Bribes, taxes, maintaining a lavish lifestyle to impress others, infrastructure and so on? I've always asked people to keep 2 sets of books if I have something like that, only "adventuring income" can be spent on adventuring gear. Mostly I do that so I can keep the business side of things more abstract, I don't want to play paperwork and accountants.
The "aristocratic" setting tends to be normal for us, because we have a living world, where the new characters are the children of the semi-retired high-level characters. In principle, the level 1 characters have virtually infinite gold.
Our solution is all magic requires "attunement". There is a limit to the number of items (bodyslots) and consumables possible. High-level items are present but unusable by lower level characters. Likewise, even level appropriate magic requires the character to be in tune with the intention of the original magic item creator. So a DM can arbitrarily have magic items present that are out of sync with the intentions of the player character.
It is impossible to buy and sell magic items, because the items themselves often "refuse" to cooperate. The intention of the creator is an important theme.
In fact, despite the local ubiquity of magic items, the setting is a defacto low-magic setting, in terms of scarce access to magic items.
Regarding red tape, like paying taxes, lifestyle costs are abstract, and use the costs in the Players Handbook equipment section straightforwardly. Bribes risk criminal investigations that discredit the family.
A wand or component pouch can only be substituted if the component has no GP cost. I would say the example I gave - the object required for find the path - is an exception to the rule. It doesn't list a GP cost but it's so specific I would call it required.
The gp spell component is an ongoing dissatisfaction with commercial flavor. Moreover, the gp costs are inconsistent, and it is difficult to see a pattern in order to remedy it. Generally, instead of a gp, there is a ritual requirement to either prepare or cast it, that can only be done once per day, week, month, or year, depending on the nature of the spell, in order to prevent spamming it.
I get the expense part, for me it's the cost of the special ink and paper. If you don't do that, there would have to be some other limit, maybe the wizard has to be in a special sanctum and burn expensive incense or consume rare herbs?
A "spellbook" can be written on stones, staves, leaves, patterns within crystal, imbued into furs or feathers, etcetera. Anything can be magic or transmit magic. The gp requirements feel like a nonstarter.
Personally I just don't hand out that much gold and assume that enemy spellbooks are well hidden and difficult to find.
Similarly scarce, each spellbook is unique to the creator, and if a spellbook exists at all, it may or may not be accessible.