I can’t imagine using Flutter for desktop development if only because like most newer UI frameworks, it’s mobile focused and doesn’t come with important desktop widgets like tableviews/datagrids and treeviews.
But limiting yourself only to standard widgets seems to be extreme.
In UIKit for instance I typically build custom buttons by subclassing UIControl, which gives a nice “blank slate” control that has all the basics of interaction covered without the specifics of UIButton. It’s easy to build a custom button that’s as well-behaved as a standard UIButton that way.
I like headers and sections more that tables for most things anyway.
I've replaced a lot of tables in my apps with modal dialog UIs, it's nice to have them behave exactly the same on mobile and web, to not have any risk of mistakes due to filling in a box on the wrong row because the label is too far from the box, and it's just generally more pleasant for us non-ADHDers without very good native multthreading support in our minds.
Android style UI is all about encapsulation and reducing the mental context window to the bare minimum, and it really makes a lot of desktop stuff look pretty clunky and confusing by comparison.
VS Code would definitely suffer a lot without the sidebar tree view though, and it does seem like a lot of people prefer the more open instant access to everything that a table provides, rather than clicking through dialogs.
That said, I do think it's fine to have a simplified mobile-ish UI as an option, but it shouldn't come at the cost of more classical desktop style layouts for those who can leverage them well.
As a sidenote, this is part of why apps like the mac version of Apple Mail and Thunderbird have a considerable number of hardcore adherents, particularly among power users.
Degrading the experience for the majority just to make things better for the power users doesn't seem like it's always the best plan unless you're specifically targeting power users.
In the extreme case, UI for power users can result in production databases being destroyed, or someone getting served food allergens because the menu system made it easy to assign wrong items to wrong customers, or someone's entire family photo collection getting deleted by a single command.