If one stacked many Barlow lenses/used several focal extenders, what would the resulting image look like?
There is two effects which will happen:
if you magnify beyond the useful magnification you are essentially magnifying the aperture's diffraction pattern (thus the airy disk for circular apertures without obstructions). Thus you make the light sources (or rather everything) larger, but you don't add resolution - which will result in a blurry impression. That's similar to zooming-in to a pixelized image: you don't get more resolution than the pixels, irrespective of how large you make or display those. See also e.g. here for some numbers based on the f-number (ratio of aperture to focal length).
every surface in the optical path (a lens has two, a mirror one) adds optical errors due to manufacturing limits in smoothness and curvature. Additionally you add some degree of scattering in impurities within the lenses glass. Both effects reduce the effective contrast and thus the resolution slightly, too.