You can easily build complex UIs without a framework. Chrome DevTools, Chrome OS, Photoshop, Firefox and other very complex apps have been built with web components.
Web Components aren’t an alternative to React so much as a complementary technology. Yes, you can build UIs without a reactive framework but it will involve compromises in expressivity, code organization, and whatever the Greenspun equivalent is for React.
Web components do not imply a lack of reactivity. JavaScript accessors, observed attributes, and a declarative template library give you as much reactivity as React. Photoshop and Chrome are built with reactive web components.
The precursor to Web Components was XBL (NB: only ever implemented in Gecko), which was used extensively in the Firefox UI. (The Firefox product consists of literally hundreds of thousands of lines of JS powering the app UI—and many of its background components.) Gecko was doing Electron-style apps before Electron (or Chrome and JITted JS engines) ever existed.
