It is you who refuses to believe the truth...
Sure sure.
Future sight isnt permanent
What do you mean with "isn't permanent"? Future Sight works at the exact same way like regular CoO; subconciously which Luffy was later able to.
it is off from the moment of intervention until the next time user re-sees the future.
Do you remember that conversation between Luffy and Kata - before Luffy went on Snake Man - when each of them exactly knew what the other one is going to tell about?
FS users literally see entire moments in the future in several steps.
That's why they exactly know how someone is going to move at the next moment and thus they can hit them before they move properly.
During the battle, Katakuri also explained the same when fighting Luffy.
If Luffy dodges from right, he'll exactly aim where Luffy is going to dodge.
If Luffy blocks with his arms, Kata is going to aim for the shoulders.
After that Mogura incident, Luffy subconciously dodged Kata's barrage of attacks but when Luffy was going to charge for him, Kata aimed on his legs to prevent this move and thus stopped Luffy's movement.
The benefit in the Snakeman form is very low due to high attack frequency.
No, this is a fallacy.
Snake Man perfectly can scheme the re-direction of his Python - after all, it predicted that Kata was going to dodge the first hit but it redirected its Python and managed to hit Katakuri this time.
This accuracy is a prime example of Snake Man's usage of FS.