Movie actor's involvement in a film is crucial till and after the shooting is completed. This is especially important when a series or a franchise is planned. Now think what will happen when one of the actors doing a main role passes away in the midst of shooting a grand project way into the franchise, when it is in its 4th or 5th film?
The shoot cannot be stopped, nor can the film be re-shot from the first. The makers of Fast & Furious 7 faced a similar situation when Paul Walker died. Walker died in an accident in November 2013, when the production of the movie was midway.
At the time of Walker's death, apparently there were upto 350 unavoidable scenes which required the presence of Walker. 90% of the scenes were completed using shots from the archives from the past Fast & Furious movies filming.
The other 260 shots were completed by having Walker's brothers, Caleb and Cody, perform his scenes in character, only for their faces to be replaced with Walker's CGI version (Computer Generated Imagery version) during post-production.