Thank you for the article Justin!

I agree that artistic themes are important, and it's what themes should do.
However, in its current form, Gutenberg & Full-Site-Editing are evolving extremely fast. An FSE theme could add opinionated styles for buttons, write 100 lines of CSS to add just the right shadows, border-radii etc, only to see their awesomely-styled buttons become the cause of bugs in a future release of the editor. At some point, the buttons get a border-radius control, then they might get a shadows control, or even hover/active-state controls. If/when extra controls get added, themes will be able to simply define their value in the theme's theme.json file and styles will be auto-generated. Users will be able to tweak these values from global-styles in the site-editor.
So if a theme adds a bunch of styles now, while FSE is fluid/unstable, they run the risk of spending tens of hours on something that will eventually be doable with a single line in a JSON file - not to mention the opinionated styles will probably conflict with the future auto-generated styles from the editor.
For example a theme can now add customizer controls so that users select font-families. Writing that customizer control will basically be dozens of wasted hours to write the control, JS, applying the resulting CSS, loading webfonts etc... this will all be part of global-styles out of the box eventually.
And that is why I previously said "Theme authors should avoid the trap of designing an FSE theme having in mind what the editor currently does. Instead, theme authors should strive to build something having in mind a vision of what the editor will eventually become."

Until some APIs and implementations solidify, FSE is a constant learning experience and we shouldn't rush to build the next big theme... not before the foundation is stable.