Flash was terrible for producing documents, I suppose, but for highly interactive things, nothing matches it, even today (we are getting close in terms of what the web is capable of, performances are getting good, but the tooling and implementation effort/complexity is ridiculous in comparison).

I don't think it's so much a question complexity than a question of using the "right tool for the job". Web technologies were never designed for that, never engineered to be extensible. Web apps is a hack, it's terrible, and that's all because of politics.

Web technologies were designed to be extensible, for documents. e.g. HTML has had a built in templating engine and support for websites defining their own custom tags for 25 years.