This is how the default scaling looked, so you were totally correct. it is clear to see that the left one is the scaled one:
Now, this is a high res image and with a video in motion, I think the problem is not as drastic, but you do have a point.
The trick is not using sharpening, but actually changing After Effects quality settings. When we use a simple 2x upscale and then 2x downscale, it is best to use the lowest quality rescaling method. That is a simple pixel upscale.
We can enable this with this box::
If we set this to the pixelated line shown in the screenshot on both the image as well as on the comp, that is shrunk down, we get the result from my first post.
When using this setting with a perfect 2x upscale/2x downscale, this should result in a near perfect result. However, using this setting with not so perfect numbers, like a 66% scale for example, the other two quality settings do give us much better results. This is why After Effects prefers the other quality settings.
Using “Best Settings” as render settings, After Effects does handle the scaling differently. I first thought it sets all layers back to the supposedly “better” scaling method that results in the blurred image of above, but this doesn’t seem to be the case. We need to set the Render Setting to “Current Settings” and set the Output comp manually to Full Resolution, to have a perfect result.
Here is a comparison of all the different layer quality settings, rendered with “Current Settings”
We can see, the default (lower left) performs worse here. The highest quality method does in fact do pretty good, but I think the upper right image is still the one closest to the original.
Now here is the same comp with the same layer settings, but rendered with the “Best Settings” preset:
We can see that my suspicion that After Effects just assigns every layer the same quality setting is clearly wrong, as we have different outcomes here for our different layer settings. So the render setting “Best” is doing something under the hood that cannot really be explained. I came across this with other projects in the past, that the rendered image/video did not look exactly like the preview in AE. To essentially get what you see in the preview window, you need to set it to Current Settings.