Bill, I shot a D3 for a long time (I work almost exclusively with 24mp Fuji now...) and I've got to say that up to about 20"x30" it looked as good as anything I had printed that big from Kodachrome or anything else. Maybe better in some cases.
I used a variety of up-res programs and before that I used a script to automate up-resing in Photoshop CS3 (?) up to 100% in 2% steps. Or maybe it was 5%, I can't remember because I settled on Genuine Fractals an age ago. I print in-house on an Epson 7890 with their 10 ink system at 260-300 dpi. (Don't get me started on what's happening with ink these days.)
On the road I print with a little Canon Selphy at 4x6" and I'm always trying to fix the color. The Epsons are where it's at.
Man, I don't know. I really think you might be able to divide the discussion in two.
The first is technical: What do you need for best detail? So, technique aside (because that for sure is my limitation), and leaving out depth of field and content (hair is brutal)....more native pixels is always better at any given print size. So the question really is: How big do you want to print?
The second is then: How big can you print? Probably you can print a little bigger than the mathematics suggest but that will be a subjective choice. Usually (for me) a smaller, "tighter" print will blow away a big sloppy print where things are teetering on the edge of posterization or artifact.
These days I never print larger (on the Epson) than 12x18 and 24mp seems to do just fine. Even with jpegs right out of the camera although I'll sometimes go back to the RAW file and work it to where I think I remember what it looked like. Again, subjective. And it really depends on content. Large colour-field pieces are usually fine straight out of the camera but portraits usually come from the RAW file.
But here's the thing: maybe I'm a slave to content, but I often have to look at the metadata or the back of the print (only sometimes do I write stuff there, so...) to figure out what camera I shot it with. Which in itself is a sort of irrational quest - what does it matter, especially if I can't tell by looking. Yes, sometimes it's obvious what I shot with an iPhone but sometimes it isn't. Look at David Alan Harvey's work out of the favelas. Some of it is iPhone, some of it M9, some Fuji X100, I think.
Look at Alex Majoli's work with Olympus C8080 cameras after he went digital - It is freaking stellar. Granted much of that I've only seen on a screen which, unless you've got a 4K screen, you're probably not getting the whole picture - detail wise.
Lotsa moving parts, Bill. In the film world, look at Lange's Migrant Mother. Detail shmetail. She "missed" focus ever so slightly if you want to nit-pick, and that means lost detail, but who in their right mind would care? I certainly don't. The only thing that photo makes me care about is how badly we as a society have.....etc. Not a single synapse is devoted, outside of this discussion, to the technical because the content crushes it. How about Capa's D-day work. Boom. And then back to digital there is Moriyama who has made a complete oeuvre out of occasionally omitting detail.
So I guess this is a really long winded expression of what could be summed up by the phrase, "I dunno, love. Depends."
I've been lurking for a long time and really enjoy your column. I've learnt a lot.
All the best,
Shane