Just to clarify this - "Scaled (Looks like 2560x1440)" mode isn't "2k" and it isn't "effective 2560x1440 resolution" and its debatable whether it could be called "interpolation" (which usually means making up data that isn't there). It is (as @thirdsun did go on to say) 5k downsampled to 4k and, as such, carries a lot more detail than an actual 2560x1440 display would. . "Looks like 2560x1440" just means that the system fonts and icons are the same physical size (i.e. in mm pr points) that they would be on a 2560x1440 screen.
Yes, high-contrast edges like text look slightly "softer" than they would otherwise, but frankly that's a matter of taste - its similar to "anti-aliasing" which is often done deliberately to make fine detail look more natural and less pixellated.
So, running a 4k display in scaled mode is a perfectly reasonable proposition for day-to-day use (you can always change mode for games or 3D work) and "Looks like 2560x1440" mode is about perfect, for most people, on a 27" 4k display.
However: the concern is whether the base-level Intel iGPU - with no dedicated VRAM (needed for off-screen 5k buffers) - in the Mini is really up to this - and there's no reason to expect that it would be: Intel designed it as a better-than-nothing option for entry-level desktop PCs that would mostly have space for PCIe GPUs, and would be running Windows which doesn't do that sort of scaling.
That's why the lack of a dGPU in the Mini is an issue - its one thing paying $700+ for an eGPU that outperforms anything else in a Mac at serious 3D work or OpenCL, it quite another thing needing that to run a decent, retina-class display or two without restrictions.
