With today's flicker-free displays, framerates up to several-thousand frames per second would still be noticeable improvements.
We haven't seen diminishing returns yet. Every time you double the framerate starting from 30 FPS is a very noticeable difference in smoothness, responsiveness/latency, and motion clarity.
Now you could argue that you don't necessarily need framerates that high to achieve very smooth motion, but it requires that we go back to driving displays in an "impulse" fashion that strobes the image once per frame to improve motion clarity, rather than being flicker-free.
24 FPS only ever looked smooth with a film projector that used a single shutter.
As soon as double or triple shutters were used, motion in film at 24 FPS stopped appearing smooth.
And now that our televisions do not flicker at all, motion is worse than ever before - if you are not using interpolation.
Making displays flicker-free requires that the framerate is
significantly higher than before to achieve smooth motion.
Caution: low framerate flashing images.
Here is an animation which should demonstrate this effect.
It assumes that you are viewing it on a flicker-free display - most LCDs, OLEDs, and other non-plasma or non-CRT displays.
Both circles are moving back and forth at the same low framerate.
The upper circle is being displayed without any flicker.
The lower circle only appears every 6th frame, which causes it to flicker.
If you cover up each circle with your hand, you should see that the lower circle appears to be moving considerably smoother than the upper circle.
You may also see ghosting or double-images in the upper image, which are not present in the lower image.
That is the effect of low image persistence, compared to full-persistence or "sample-and-hold" displays that don't flicker.
It's the reason why motion even in 60 FPS games is not as clear or fluid on today's LCDs or OLEDs as it used to be on a CRT display.
Yeah, about that...
Don't blame framerate for a bad movie.
It looked worse at 24 FPS.
They should really move to 60 or higher.
Moving to 60 would be smart, since nearly every television can display a 60Hz signal.
Pretty much nothing officially supports 48, though some might sync to that if they support 50Hz (PAL) and have a wide enough tolerance.
Anything else requires new TVs, AV Receivers, and potentially other equipment.
Give it a week or two to adjust, and you may find that you have a difficult time going back to watching TV or movies without it.
Interpolation is imperfect - it
is going to introduce artifacts with complex motion.
However I would argue that even with those artifacts it's far better to watch a movie with it enabled than not, unless your TV is doing a really bad job of it.
I have set up a test before which displayed a 24 FPS movie on a CRT that is actually refreshing at 24Hz - or you can use higher refresh rates like 72Hz with two black frames inserted between every image for an effective 24Hz.
It flickers a lot, but looking past that, motion ends up perfectly smooth.
It's so smooth that when you switch from 48Hz to 24Hz it looks as though someone switched on interpolation - except an analog CRT can't do that.
As soon as you start to double or triple flash the image for 48Hz or 72H though, that effect disappears completely and you are left with the awful juddering motion that everyone associates film with today.
Don't believe anyone that tells you interpolation is not how film is supposed to look - since displaying 24 FPS with a single strobe per frame on a projector or a CRT results in motion that looks very similar.
Interpolation is just a bandaid to help with the fact that movies have stuck to 24 FPS for so long, despite the move to higher refresh rate displays, and now flicker-free displays - when the framerate should have increased with each of those changes.
