But unless the 'grabee' doesn't do anything, someone grabbing one foot from someone is likely using both hands (and most of their upper body) to attempt to control it - and there is a second foot/leg out there free to cause a lot of damage if they're willing.
It's not a 'one and done'/'game over' scenario by any stretch of the imagination if the party is even a little motivated.
Though I suspect most folks in this thread have no exposure to an actual fight beyond watching a video somewhere, so....