Warner has retired from Test and ODI but will continue to play T20 cricket, as per my knowledge. He is no doubt one of the greatest opening batsmen produced in the history that can play test, ODI and T20 format. He play test as genuine test player while in T20I he is a solid hitter like a young player. Aussies have a good domestic setup and they will produce some other great opener to replace Warner.
David Warner will continue to play T20 as well as various domestic league tournaments. Many national cricketers are hoping for a form like David Warner's farewell to the Test and ODI format. He could have served the Australian team for another four to five years if he wanted, but he felt it was the right decision to retire earlier. If David Warner is to catch up, he has given the opportunity to the youth, he has retired. David Warner will be truly missed in ODI and Test cricket especially when it comes to big innings, surely every Aussie fan will remember him.
To be honest I agree with @MusaPk. In the three tests that Pakistan has played in Australia, I think Warner got about 5-6 chances in the 6 innings that he played in these 3 tests. He scored so many runs after he got them second chances. He would have probably scored half the runs that he scored in this series than he did after getting all the chances. Shaheen Afridi and Jamal were getting the fielders the chances in the early stages of the match with the new ball and Warner would have gotten out early on so many occasions.
But despite all that he is still an Australian legend I think it will be hard for Australia to replace him.