Given you can live a pretty good life for a pretty long time with far less than that, I think I stand by my statement.
But I don't accept that as given: See below.
I'm not talking about the money needed for active PCs. I'm talking about "I want to get rich enough to go do other things without worrying about it ever again." Whatever one can say about 3e, the amount of money gained was easily burned by an adventuring PC in an ongoing fashion, but that's got nothing to do with how ridiculously wealthy they are in a mundane sense.
The assumptions I'm making are:
1. The median income for characters of Nth level is N-squared gp per day. That applies to NPCs and NPC-class NPCs as well as to PCs and retired PCs. I drop the conceit that "1 gp is big money! Enough for an average 1st level NPC to live on for a week or more!"
So a typical fifth level character will have an income of 9125 gp per year. And those characters aren't all, or even mostly, adventuring PCs or PC-class NPCs. They're mostly things like 5th level NPC Experts (wealthy merchants) and 5th level NPC Aristocrats (members of the landed gentry and minor nobility).
2. The median income for the population overall will be between 1 gp/day (1st level) and 4 gp/day (2nd level). Or between 365 gp and 1460 gp per year.
That means a typical fifth level character has an income of about 10x that of the global median income. Rich, but not outrageously so - very roughly equivalent to a modern American making a salary that's well up in the 6 figure range.
3. Income from passive, abstracted, guaranteed-not-to-be-used-by-the-DM-as-a-plot-hook investments will return 2% per year.
That calls for 456,250 gp in such passive investments to produce 9125 gp in income per year, which is where I get my "close to half a million gp" from.
Now the "by the book" D&D background material tries to force the contradiction of D&D being simultaneously a Realm of Golden Glamor & Wealth
and a Dung Age Place. If you really want to keep the second part, with its conceit of "1gp is enough for a 1st level peasant to live on for a week, 10 days, or even two weeks" then that calls for cutting my assumed figures by a factor of 10. And then yes, the wealth required to retire "rich" is cut down to something obtainable by a 5th level character with only a little more wealth grubbing than normal for 5th level adventuring.
But I find "1gp is a lot of wealth, in the mundane sense" to be an assumption that unnecessarily makes already-crazy D&D economics even more so. My judgment, my aesthetic sense - my taste, if you will - says to drop that assumption and so make things slightly less crazy.