Maybe it's targeting a different use-case, but these things (at least on the Web) appear to be more-heavyweight and less-capable than the things people were doing 20 years ago with Macromedia/Adobe Flash, e.g. compare the animated-GIF-like examples linked from TFA ( https://thorvg-perf-test.vercel.app/ ) to the animations and games found on sites like Newgrounds. Last I checked, the latter make heavy use of emulators like Ruffle, or (based on loading screens) 3D game engines like Unity etc.
As someone who's been out of that scene for a long time: what's the overall state of things, if I want to make long, complex, 2D vector animations? (i.e. not using a 3D engine; and not rendering to video). SVG seems pretty established; but for animation, how capable is Lottie? Does anyone still use SMIL (outside of DVD menus)? Am I better off "rendering" to a big pile of JS + CSS transitions?