We're not too far away from SIMD, Atomics and SharedArrayBuffer as well as OffscreenCanvas in the browser (they're all available behind experimental flags today). I can definitely see a newcomer building a web engine from the ground up and beating Unity/UE4 in performance AND productivity for not having their overhead. Its a huge undertaking but nothing impossible.
What I missed the most doing WebGL work however were asset pipelines. Open-source engines barely support DXT compression, normalized integers and whatnot. We ended up writing our own (very crude) CLI tool to compress DXT1/5, ETC1 and PVRTC as well as a KTX parser to load them at runtime. I'll see if I can make them open-source - they're still a bit tied to our custom in-house webgl renderer.
We went with ImageMagick and PVRTexToolCLI driven from a node.js script. I'd go with GraphicsMagick now that I know about it; IM doesn't yield good DXT compression quality.