When one upgrades one's camera to one with more pixels, keeping the format size the same, the reason is presumably to enable one to make larger prints which are still acceptably sharp.
Indeed. Let me put it this way: a sensor with higher pixel count through more, smaller photosites on the same sensor size gives the option of seeing more detail in the image, and that option comes with less DOF. So when one aims to use the greater detail and also wants adequate DOF, this must be accounted for in choice of aperture: smaller apertures are needed. And thus, getting the maximum possible detail from the higher resolution sensor imposes a lower maximum DOF.
But one is never forced to accept lower DOF than is possible with a lower resolution sensor, since there is also the option of not using the extra detail, for example by printing and viewing in the same way as one would with the lower resolution option, perhaps downsampling first.
And therefore, choosing a sensor of lower resolution does not add anything to one's DOF or related IQ options: it simply discards some options for more detail combined with less DOF.
More generally, I still have not seen any good evidence or arguments that higher pixel count sensors cannot match or outdo the results of lower pixel count sensors of the same size with suitable processing or presentation. The processing needed might be as simple as converting to JPEG (or TIFF) at about the same pixel count as the lower resolution sensor.
Or maybe conversion to JPEG at a somewhat lower pixel count than the lower resolution sensor, since in JPEG's of equal pixel count from Bayer CFA sensors, those from a higher pixel count sensor can have a detail advantage, due to less loss of resolution in the demosaicing interpolation.