There has never been a better time to make an EX movie. If Avi has some good business sense he will put his ego and differences with Sly aside and work things out to get EX4 going. This is probably his best window of opportunity to leverage the star power momentum building up around some of the main cast members of the franchise - namely Statham, Sly and Lundgren.
Statham has seen his star power propulsed thanks to the mega success of the 8th FF, and a spin off movie focusing on his character (and Johnson's) is being widely talked about. He is now a well recognized star worldwide and should enjoy that status over the next few years with the FF franchise.
Sly is set to appear in the upcoming GOTG2 a guaranteed blockbuster. He should be getting a fair amount of publicity over the next while and his star power could keep going up. He's being talked about to play a major role in the upcoming instalment and maybe even in the Avengers series. He has once again risen from the ashes.
Lundgren was recently confirmed to appear in the next DC Aquaman and has received a lot of attention. It is still to be seen whether his momentum will be sustainable over time but for now and until Aquaman is released he should enjoy some comeback.
Now you have 3 EX actors attracting a new fan base from the FF, Marvel and DC universes - put them together in a movie and you could bank on a fairly strong amount of interest and publicity if timing is right, which should be now or it might just be too late.
So Avi what are you waiting for? This is the time, the franchise has never had so many of its cast members enjoy this much publicity and attention at the same time. Don't waste the opportunity.