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It's an exaggeration (a huge one) to say that many native Japanese cannot write by hand anymore due to technologySure, such an absolute statement is an exaggeration, and yeah, it's "debatable," but Japanese themselves seem pretty firmly convinced it's the case.
Practice is necessary to maintain a complex detail-oriented skill, and when many people are doing most of their major writing using keyboards of various sorts, and only writing by hand in very casual and formulaic contexts, there's going to be some fallout.
I think this is rather different from spelling mistakes because spelling mistakes occur regardless of the input method used (computer text entry of western languages is much more "literal" than computer text entry in Japanese). If you write in English at all, regardless of input method, you're going to exercise your spelling ability (even if using a computer lets you spell check, there's still a feedback loop). Japanese text entry using a computer, on the other hand, is completely different than writing by hand; they exercise very different abilities.