Supporting this: if you are aiming at preserving the “film look”, including grain, dirt and tiny scratches, and if you can afford to handle the corresponding increase in processing time and storage space, go for the highest resolution you can handle. This will also give you more latitude for possible newer (upcoming) ways to post process your files. One case in point is the upcoming availability of HDR-formats and -devices, which most film scanners, distribution channels and end-used devices currently have a hard time to handle at this point in time.
Even if you do not aim at preserving the original film look, the scan you are doing of a certain film today might be the only scan possible, as certain film emulsions have a severe tendency of color-fading. That is, the scan you do today might not be reproducible in the near future. Again, a reason to go higher with scanning resolutions.
Some other remarks: the pure image content of a Super-8 film is substantially less than what might be appropriate as the scanning resolution. From the limited experience I have with old film stock, I think a good average estimate of the image resolution of Super-8 film could be around 720p. The example you posted above shows, if watched in 4k, scratches and dirt in all their glory. But the actual image stays fuzzy; any picture element I am seeing is stretched out over several pixels. Clearly, the films original image content is over-sampled to a noticeable degree. That you do see a difference between watching this footage in HD or 4k is probably related more to the variances of the compression algorithms at work at different resolutions.
Note also that the SMPTE-test film shown above most probably was not produced with standard film stock. Most certainly, a higher resolution film stock was used for this. As your MTF plot of Kodachrome stock above shows, even 100 cycles/mm were out of reach for one of the sharpest films that consumer could buy in the old days.
Summarizing: one should scan (for archival purposes) with the highest resolution which is deemed to be affordable. For distribution channels like YouTube or so, it might be also be advantageous to go to the highest resolution possible, just to preserve the existing image definition. For other distribution channels, much lower resolutions might be possible, as the native resolution of Super-8 footage (combined camera and film resolution limits applied) is most probably less than HD.