I think perhaps what was lost is mostly this: Macromedia. They had a knack for making content creation simple. Flash was just one of the results of this: It let people create seemingly-performant, potentially-interactive content that ran almost universally on the end-user computers of the time -- and do it with relative ease because the creation tools existed and were approachable.

Macromedia also provided direction, focus, and marketing; more of the things that allowed Flash to reach saturation.

Someone could certainly come up with an open JS stack that accomplishes some of the same things in a browser on a modern pocket supercomputer. And countless people certainly have.

But without forces like marketing to drive cohesion, simplicity, and adoption then none of them can reach similar saturation.