"I never developed with Flash but my understanding is that "modern web" can do everything Flash was used for."

True, it can, BUT what's lacking in the total arena is actual authoring and development toolchains, which is what Flash packaged all in one single bundle.

I'm not trying to install 4 or 5 different things, along with all of their dependencies, just to make some 200MB thing that could have been done in 20MB and one program with Flash.