I'm going to assume you don't want Excel to look the same on your desktop as on your phone. But even between Windows and macOS, I find it hard to believe you don't want at least a reasonable baseline of appropriate native behavior.
Are you primarily a Windows user and just want the app on macOS to look and feel like the Windows environment you're used to—essentially wishing there weren't different platforms to begin with, but otherwise dodging the issue? How would you feel if you encountered a macOS-style open/save dialog while working on Windows? Or if an application tried to attach its menus to the top of the screen instead of the top of each of its windows? Or if it responded to Ctrl-C by cancelling an operation instead of copying what you had selected? Or if the window close button was in the wrong corner?
You're just prejudging the answer. Who wouldn't want reasonable and appropriate? The issue is that a lot of these platform-specific defaults are bad unergonomic legacy
Case in point: "Or if the window close button was in the wrong corner?" That would be great to have on any platform since it's a common UI mistake to clump categorically different buttons (non-destructive minimize and destructive close) together, thus raising the cost of a misclick
That's the main issue with your argument - yes, familiarity is a UI benefit, but the net benefit of following a UI convention depends entirely on said convention.
I agree that there usually needs to be some level of OS consistency - using system file dialogs, keyboard shortcuts, etc. - but I don't think it extends to aesthetics.
I used to think like that because it's very much a core tenet of the mac/apple ideology (it's basically a form of narcissism at the level of a brand).
But the actual reality is that the OS does not matter that much. The application is actually what's useful and exactly why we are even using the computer in the first place. The OS is just a middle man that we can't really cut out but should make itself as transparent as possible.
It follows then that the UI should be designed around the needs of the application and that it should be translated/transposed/copied across platforms regardless of the primary look of the platform. And this is exactly the problem with Apple stuff, focusing too much on superficial aesthetics at the expense of more substantial usefulness.
For example, if Apple hadn't decided to completely re-skin their whole office suite to make it "mobile ready", maybe they could have worked on the actual feature set and performance (awful in both cases against the offering of Microsoft).
Ironically, when Apple ported iTunes to Windows, they did a perfect copy paste, going through the trouble of creating their own UI framework specifically for Windows. So, it seems that their requirement/demands only applies to others, never themselves.
Are you primarily a Windows user and just want the app on macOS to look and feel like the Windows environment you're used to—essentially wishing there weren't different platforms to begin with, but otherwise dodging the issue? How would you feel if you encountered a macOS-style open/save dialog while working on Windows? Or if an application tried to attach its menus to the top of the screen instead of the top of each of its windows? Or if it responded to Ctrl-C by cancelling an operation instead of copying what you had selected? Or if the window close button was in the wrong corner?