FIFA legendary mode in Ashes Cricket would just be the AI hitting 6 off every ball. That would not be fun.
FIFA legendary mode is rigged AF. They know where you are going to pass it, know where you are going to run to, shoot to, its not an organic experience. Challenging yes, but I wouldn't call it realistic or a shining like of organic AI.
Advances in AI haven't kept pace with most other things in tech. The reason is because it's incredibly difficult, and those that have mastered it will keep it under lock and key to make as much money as possible.
I feel like a good solution would be a cloud bases one, where the game records what a human does against balls pitched in certain areas and then does performs the same shot in the same ratios. That way it would be like playing against an average human. To scale the difficulty, scale it more towards the most successful shot. I don't feel like it would be that difficult to do these days, I do a lot of big data stuff at work, but capturing the data and storing it is probably the biggest challenge.