I doubt many advocates for htmx (or Hotwire) believe React is being abandoned right now. However, they do represent a serious challenge, because they’re not just marginally faster or more ergonomic like, say, Preact or Svelte - they represent a complete paradigm shift from how we’ve been building Web apps for the past ~decade. Being able to have over 80% of the interactivity you can get with React while expending perhaps 20% of the effort (no build step, no client-side data modeling and routing, no client-side state, easier for server-side oriented devs, use a language you’re already proficient in, reduced cognitive load, etc.) is huge.
Many pioneering technologies are initially met with skepticism. A decade or so ago, jQuery was the clearly dominant client-side library. Very early adopters of React and its siblings were swimming against the tide, but eventually they made jQuery much less relevant.
I’m not saying htmx will definitely become what React is today, but the belief that a dominant technology or paradigm is safe from disruption has been repeatedly proven wrong.