The effects in some of the demos simply kill any video encoder, only way to properly experience them is by running or emulating locally. Not that crystal sharp upscaled square pixels on modern display or poor quality CRT filters are very representative either, but it's likely better than blurry mess you get when video encoder tries to deal with high frequency details or random noise from procedural effects.
High fps is probably a little more rare than HiDPI, but also becoming more common. It's already common in phones.
A workaround for modern hardware is frame doubling then blanking every other frame (software black frame insertion). It halves the brightness, but modern monitors generally support higher brightness than CRTs so this just brings it back to the authentic level. It's possible with FFMPEG filters, although I don't remember how I accomplished it when I tried it years ago.
The importance of CRT flicker is one reason I greatly prefer DOS demos to C64 and Amiga demos. Those were mostly designed for 50Hz PAL CRTs, so the flicker is far more annoying than 70Hz VGA.