The neat thing is that 25fps content played out at 50fps doesn't use a lot more bandwidth than 25fps played out at 25fps because it only has to update every second frame, assuming identical noise between frames.

And then you can go to field-per-frame 50fps footage when you need to say "camcorder".

It takes twice the file size to double the framerate without reducing visual quality of the original frames, whenever I try, with HEVC, AV1, etc. I can take a lower framerate video, run some software of constant size to interpolate the frames, and play back at a higher rate, but in practice I can't save that higher framerate video to a new mp4 file with small or constant size increase. It's got to be about linear.