Without a screenshot that UI will look very bad. Its a decision to improve UI and not to improve the Product.
iOS store
macOS store
Windows store
Steam
Epic
Play store
Discover
(Gnome) Software
All these support screenshots. It's pretty reasonable that after a decade of millions of people installing apps this way they would want to see what they're getting into.
Yeah something generic would help, but manifest should still absolutely support the idea of screenshots.