If they keep doing sequels eventually Harrison Ford will have to be replaced. It's hard for me to see anyone else as Indiana Jones. But there was a time it was hard for me to see anyone else but Sean Connery as James Bond. Then I couldn't imagine anyone else but Roger Moore as Bond, you get the picture. I can see Ford doing maybe one more movie. After all they can do a lot with makeup, stunt men and editing.
I'd quite happily watch a new Indiana Jones actor if it means some more movies.
No reboots necessary.
Fill in the gap between the end of the 'Young Indy' series and the first (chronological) appearance of Harrison Ford in Temple of Doom.
...which would be 1921 - 1934 if I'm remembering right (and I'm probably not)
If its set in say 1928 to 1933 you could do 3 movies with a new adult actor in the Indy costume