Oh I don't know, it's always possible, not every wizard is financially smart. Harry isn't and up until the end of Battle Ground he was a member of the White Council. Now he might inherit from Eb, but at least on the surface he doesn't appear to be financially rich either.
Yeah,
Harry isn't very smart that way... but Jim has said more than once that the WC "financial wizards" are producing a LOT of wealth, and that individual wizards of the Council often invest a bunch of money with the WC investment arm.
It's just that Harry's actively
stupid about money... "sockfull of diamonds" should have gone 90% to WC investments; hell, to *any* investments!
He could literally have lived off the interest for the rest of his life... tho expensive projects (like Svartalven items) might need him to dip into principal.
And of course, the whole "Castle Upkeep, taxes, etc" issue is just fantastically expensive, not even "sockfull of diamonds" carries that very far! TBH, I'm not sure Jim thought through those "castle consequences" properly...