SPOILERS

The boxcutter will show up in the last Shiori episode. Any earlier and the purpose of the scene would not be achieved; it is of paramount importance in the construction of Shiori’s story because it establishes that the persona that we have been privy to thus far - the sanguine and apparent acceptance of the inevitability of fate - is a facade, thereby establishing the basis on which Shiori’s miracle is achieved.

Shiori’s famous line about miracles not happening is presented in Kanon as an irony; one of the series’ primary tenets is that miracles do happen if they are desired badly enough. This is why the boxcutter scene is as important as it is, and can only be showcased in the arc’s conclusion; it establishes the reason for the inevitability of Shiori’s demise as her own acceptance of that demise and, at the same time, details the importance of Yuuichi in Shiori’s life. Shiori in the game is a character who is essentially sick of life until Yuuichi comes along and begins to patch things up for her. He becomes the basis of her miracle; the individual who teaches her to fight and eventually survive.

Without the boxcutter scene, all that goes out the window, and Shiori’s story loses most of its depth and its impact - her story was always written with her survival in mind as its true ending, yet it comes off as overbearingly cheesy and unsatisfying unless if it happens without that justification detailed above. Without the boxcutter scene, Shiori’s story is little more than Yuuichi’s observation of her strength. Without the scene, the viewers are never able to penetrate into the depths of her thoughts and her psyche, and her story ends up unsatisfying and incomplete. It’ll show up in the end. No scene is more important to Shiori’s story than it is.