They would be dumb to cast anyone else other than Ryan Reynolds. Like, that would be actual corporate suicide, so they would never do that.
Hugh Jackman, on the other hand, while he was fantastic, had his time and went out on an amazing high note in Logan. Let's see someone else wield the claws, I'm excited for that.
Side note, is that logo real? I don't ever recall this or the FF logo being revealed, just that they mentioned those properties would be coming to the MCU in the future.