In 15 years, technology quite improved. I'm gonna sort of compare two hypothetical cameras with specs representing the technologies of their eras, in a very specific scenario. What are the differences between a old crop sensor 6MP and a modern full frame 45MP DSLR when printing a 10x12?
The most obvious difference seems to be megapixels count of course.
Speed, frame rate, buffer size, AF, user friendliness, stabilisation, ergonomy: all of that immensely improved and made the process of taking photo more efficient.
But when it comes to image quality, especially when the photo is printed, what parameters actually changed?
The typical hi-end 6MP DSLR in 2004 produces photos that quite small: 3000x2000 pixels. That allows to print 8x10 at 300 dpi, or easily a 10x12 at 240dpi. A full page in a magazine is definitely not an issue.
The typical hi-end 45MP DSLR in 2019 produces photos that are definitely not small: 8200 x 5500 pixels. That allows for poster size prints: 24x36 at 240 dpi. Therefore it allows absolutely any print size that is smaller than that.
Since we can't compare a 6MP 24x36 print with the same print from our 45MP camera, we'll have to compare the largest print both can produce.
And here, something interesting happens: to print a 10x12 at 240 dpi, we're going to resize the 45MP photos by 35%. And that is equivalent to pixel binning. For the sake of simplification, a block of 2.7x2.7 pixels will become a single pixel. When it comes to noise this has huge good consequences.
Obviously, I'm considering here a photographer who knows how to frame and doesn't need to crop in post. But even if you need to crop a little in post, the 45MP advantage with pixel binning is still significant.
Some very simplified maths tell us that at base ISO, pixel binning improves SNR (in dB) by very roughly the width of the pixel binning. (It's of course actually a "little" bit more complicated than that, because the more you push the isos, the more the SNR degrades, but at the same time, the improvement of SNR due to the pixel binning is more and more noticeable. Anyway, for the sake of simplification, at base ISO, the resulat is close enough.
At base 100 ISO (we'll consider that real ISO and manufacturer ISO are the same), on a 18% grey chart, our hypothetical 45MP camera has an SNR or 45 dB. With the binning factor, the SNR is now 48 dB.
At the same time, our old 6MP DSLR has an SNR of 36 dB. Even if it isn't actually bad, a 12 dB difference in noise is something that litteraly jumps in your eyes. On our 45MP camera it is the difference between shooting at base ISO and 1000 ISO.
Since noise kills the perception of details, shooting a modern 45MP camera is a huge advantage in sharpness.
DR is barely affected by pixel binning, so I will not take it into account here. The old camera shouldn't be ashamed of its 11EV of DR at base 100 ISO, but the new 45MP is literally killing it with 15EV of DR.
That will not exactly show on our print, since the best quality prints can barely show 7 to 8 EV of DR.
Of course you can do more light correction in post with the 45MP camera, but if you know how to expose, 11 EV, or even less, is far enough in most scenarios.
It's all about pixel pitch. the 6MP crops sensor camera has a pixel pitch of 7.8 µm and the 45MP has a pixel pitch of 4.3 µm that becomes 12 µm because of pixel binning.
On the old 6MP, diffraction will kick in around F12 and will become a real problem around F6.
On the pixel binned 45MP, diffraction will kick in around F18...
The newer one will allow apparently more depth of field, but if you take the crop factor into account, it's basically the same.
When it comes to printing up to 10x12, shooting the modern 45MP offers significant improvements in noise, details and sharpness.
Here is an example: I stripped all the exifs, and had to crop a little to make the framing match, but my point is still valid. Try to not focus on the lens differences (geometry, fall off in the corners, slightly different field of view that I tried to correct by stacking the two images). One of the two images is cleaner, and sharper. When you print it, the difference is a lot less obvious, but still noticeable when you know there is actually a difference, and you look at the print with a magnifier.
That is not a fair fight. Beside image quality when you resize to print and match the 6MP, you can obviously do MUCH more with the modern 45MP camera: you have virtually almost unlimited crop possibilities, better image quality, better handling, better AF, better glass, better everything. But it's also much more expensive than the old one. You'll have to spend a few grands to buy that great modern DSLR and the good glass that comes with it. At the same time, for less than 200 bucks, you can have that old 6MP camera and some good glass.
The best good print you can get from an old 6MP DSLR is 10x12, the best excellent being 8x10. You won't have good low light capabilities, zero crop options, and you'd better expose correctly when you shoot. In the end, if you print the same landscape with those 2 cameras in 10x12 (240dpi), the results are VERY similar, and absolutely no one can tell which one was taken with the old or the modern camera.
Of course, you don't only shoot landscape, and sometimes you want to print bigger, or need fast AF, frame rate and big buffer to shoot actives subjects, or you need to shoot in low light, there is no denying the modern DSLR is a HUGE improvement over the old one, and this improvement is definitely worth the price, if you can afford it.
If you can't? well, there is no reason to be ashamed with an old 6MP DSLR.