Full Idea
Lewis denied that the grandfather paradox makes time travel impossible. Time travel does not entail that it is possible to kill your grandfather. Maybe the traveller fails in their attempt every time they try to do it.
Gist of Idea
Lewis said it might just be that travellers to the past can't kill their grandfathers
Source
report of David Lewis (The paradoxes of time travel [1976]) by Baron,S/Miller,K - Intro to the Philosophy of Time 8.2
Book Reference
Baron,S/Miller,K: 'Introduction to the Philosophy of Time' [Polity 2019], p.202
A Reaction
The problem with this answer is that killing the grandfather is only an extreme case of making any change in the past which ripples forward into the present. Virtually all changes by the time traveller would be impossible (though not contradictory).
Related Idea
Idea 23020 If a time traveller kills his youthful grandfather, he both exists and fails to exist [Baron/Miller]