To anyone in their 20's, Tom Cruise looks - and is - old. Everyone in their 50's and 60's playing their former roles in the Deadpool movie looks old. Blade is not supposed to age quickly. They need an actor in his 30's who can play into his 40's without looking significantly different. Ali is 51 NOW. If they don't film this for another four years, he'll be 55. By the time a sequel comes around he'll be 60. And they'll need to reboot. With a younger actor. It's pointless casting at this stage if they want to start a franchise.
I mean, James Marsters was a fantastic vampire, but people want a Spike who looks like this:
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Not this:
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Both are great looking for their age. One is ready for action. One is ready to collect Social Security.
Ali would be great in the mentor role. And if he shows up onscreen at all, it's not going to be as a young daywalker.