In all honesty, if Messi decides to play until 2026-27, the Summer of his 40th birthday, I reckon he has a good chance of reaching 1000 goals - surely this would cement him as the greatest player ever.
He currently has 656 goals to his name, and the 2018-19 season isn't even over - there are still up to 24 games left. I believe that Messi will at the very minimum score 16 more goals this season, which would increase his tally to 672. Now, he would "only" need to score 328 goals to reach a 1000, but seeing as he would have 8 season to do so, I think it is doable.
31 - 2018-19 - 55 goals - 672
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32 - 2019-20 - 52 goals - 724
33 - 2020-21 - 49 goals - 773
34 - 2021-22 - 46 goals - 819
35 - 2022-23 - 43 goals - 862
36 - 2023-24 - 39 goals - 901
37 - 2024-25 - 36 goals - 938
38 - 2025-26 - 33 goals - 972
39 - 2026-27 - 30 goals - 1000