I "got" the argument as such, but I do not buy it. If the static upscaling algorithm (working with fixed target- and source- sizes, and therefore also a fixed scale factor (1.5), which one would hope it optimised for), somehow makes a "bad", or "OK" image look worse than it started out, it would do exactly the same to a good one -- There are no extra pixels to draw from; Those are already "baked in" to the delivered ones. The "Shit in: Shit out") principle is true enough, but so is generation loss, and a better antialiased source image could at best "smooth over" the blemishes, on account of being softer to begin with, whereas normally with a decent scaling routine, scaling up would introduce some softening, as it preserves proportional uniformity.
The 5k meanwhile, would still get an image size that is optimal to it, with or without supersampling, and benefit in one manner or the other, either way - looking every bit as much better than OK with higher supersampling as the 8k, regardless of its blockier resolution, and accompanying SDE, and other weaknesses.
If we were talking e.g, say, simple nearest neighbour scaling, anything other than integer scale factors would introduce periodic distortions, as some pixels are doubled, and others are not; Sharpest possible outcome - no softening anywhere, but the downsides are inevitable.
Now, one could pre-arrange spatial distribution within the sent bitmap, in software, to overcome (or rather: "play to-") the perfectly known and predictable deficiencies in the on-HMD upscaling algo -- kind of like we already have for lens distortion, but I wouldn't count on that.
I'll say this: If (and that's a big "if"), the transport image is fully utilised, and "blank spots", that are not visible through the lenses, are used to carry content that is then scaled to within the visible areas, then we're looking at a way to carry more information to the 8k (...and especially if the scaler could treat different regions of the picture as having different resolutions), because then you'd be utilising more of the resolution of the two 4k display panels, whereas the 5k does not have the physical pixels to put such extra information into, and would have to throw it away (...or scale it down, if it could).
Oh man... I ended up armchair "lecturing" an experienced software developer... I'll crawl back under my rock now.