If they didn't have a problem replacing Robin with Katana, I wonder why they didn't also replace Alfred with a character more in line with what they needed.
Because they didn't have to. Alfred Pennyworth's military/intelligence background has been an established part of his character in the comics for decades, a source of his skills in disguise, deception, and field medicine which have often been useful to Batman. Many stories in the comics have portrayed Alfred as a skilled fighter who was past his prime but still able to put up a good fight when he needed to. Some other screen incarnations of Alfred have had an intelligence or military background themselves. Batman: TAS
explored Alfred's history with British intelligence in its final episode, "The Lion and the Unicorn." Michael Caine's Alfred in the movies was explicitly a military veteran.
True, this version is playing up that side of his character more, but that's just a shift in emphasis, no more revisionist than earlier screen versions that ignored that aspect of the character. And since earlier screen Alfreds haven't developed that side of his character much, I'd say we're overdue for one that does.
Besides, what really defines Alfred isn't his job, it's his relationship with Bruce. "What they needed" for this character was a surrogate father figure and aspiring guardian for Bruce and a skilled assistant for Batman, someone who worries about his safety and is able to confront and challenge him about the risks he takes. There is no possible way that Alfred Pennyworth is not the right character for that.