Event loops are also non blocking. That’s literally why JS is non blocking. But event loops and callbacks are extremely hard to scale and maintain and keep non blocking. That’s why async/await is a more powerful abstraction - you don’t pretend I/O is this blocking thing, you interleave other work while it’s being done, and you don’t get impossible to follow callback hell. VS6 suffered from non responsive hangs all the time because some developer forgot to offload something that turned out to be compute heavy under certain conditions.

Also, the parts they couldn’t make non-blocking (eg file reads) were precisely where VS6 would shit the bed and hang the entire UI trying to open a large file.