This is not a React alternative. It achieves the same end result as React, it's smaller, and it's probably faster in some situations. But those are things that pretty much every app shouldn't need to care much about - every framework is fast enough to nail 60fps if you're using it properly, and few of them are so bloated that they add more than a couple of percentage points to the page weight. 3KB is definitely better than 30KB, but if your main bundle is 3000KB your users aren't going to be impressed when you switch. Clearly this varies with the audience you cater for too - a B2B app with 500 users can afford to add a bit more weight to a page than a B2C app with 200,000,000 users. We need to choose our tech accordingly.

What most JS developers should really care about is the ecosystem, the tooling, the documentation, the code quality, and the testability of their chosen framework. React really shines here. There's a ton of good libraries, there are some superb tools, the docs are very well written (especially the new beta docs), React's core code is accessible and readable, and (mostly thanks to Kent Dodds) React apps are very testable. A React-based app can be (but too often isn't) written well, be mostly bug free, be fast, and be easy to build a business on.

This is why Espresso.js isn't an alternative to React. A dev using it just won't be as capable as a dev using React, because the surrounding tooling isn't there. React isn't just the code that gets bundled in. It's everything else as well. Libraries that claim to be smaller and faster but that lack that quality of dev experience should really concentrate on improving that rather than shaving another 1ms or 0.1KB off the load time.