> Can someone explain to me why HBO isn’t spending $100M to recut season 7-8 and maybe reshoot a few new dialogue scenes while the actors are all still available?

Because the actors that matter aren't available any more, and neither are the sets, and COVID would complicate production, and because you grossly over estimate the value it would have even if none of that were a problem. It's done, the world has moved on. If there is more money to be made on the franchise, the ROI is going to be in other series based on the property (of which they have one in development), not tweaking the old one.