These movies have been mapped out years in advance, including pre-production work. So yes, it is. Kang Dynasty would have to basically get a page 1 rewrite, thus delaying both it and Secret Wars likely by at least a year. Plus scrapping the millions already invested into it. Then you need to invest millions into rewrites and reshoots and such on projects already either filming or close to filming. You simply cannot just toss everything and start again so easily. That isn't how filmmaking works. All this would cost hundreds of millions across these various projects. Know what's cheaper than that? Recasting Kang. That will only cost you some of Majors salary on his contract, whatever the new actor will cost, and some budget for new costumes made for the new actor. Easier, cheaper, more logical.