i don't think most people remember how bad Javascript was, how limited web API were, and how different browsers implemented totally different flavors of JS and DOM and standards were weak.
Flash pushed the web forward and demonstrated that it could be used to deliver real desktop like applications,videos and games, not just text and images.
Obviously it also came with a catch: the player was proprietary and full of bugs and security holes.
Ultimately, Web API progress also staled when Flash died. You still can't mesure canvas text properly in most browsers and API such as file system and WebSQL were dropped, which is a horrible mistake IMHO.
Java (and RealMedia) did that first. The thing that Flash brought was good tooling for creators and not just programmers.
For an even more thorough history of Flash, this is a great (and relatively quick read: https://code.tutsplus.com/articles/a-nostalgic-rummage-throu...
My memory may be faulty so I'll bow to anyone who has decent sources.
The Java plugin was a pain. The runtime took somewhere on the order of 10 seconds to load, and the browser was completely unresponsive during this time. This was also the era before CSS, so if you wanted animated buttons on your web site, you had to use Java. Imagine loading a page, and then seeing a column of gray rectangles that freeze your browser and eventually resolve into dithered (“web-safe” color) buttons. Not a great experience.
Real Media only played video and audio. QuickTime played video and audio and VR snapshots. There were also plugins for specific types of content like VRML.
I feel that Flash took off because Adobe spent lots of money pushing it everywhere, but also because it ran better than Java. The runtime was smaller to download than Java, it started up much faster than Java, and its vector animations played very smoothly compared to the alternatives of the time.
I hated flash btw (designers were really creative with it), but in the end it pushed JS and DOM forward, so I guess I should be thankful. :)
Edit: Adobe also killed 2 other brilliant Macromedia products, right after acquisition, Fireworks and Freehand.
But Adobe did not ignore Flash. They may have mismanaged it, but they did invest in it, and tried to push Flash into places it does not belong (AIR, and especially PDF). They also tried to turn ActionScript 3.0 into ECMAscript 4.
https://www-archive.mozilla.org/projects/tamarin/faq.html
Now that the Flash runtime is becoming banned, Adobe renamed the Flash Professional content creation program to Adobe Animate, and made it able to export to some flavor of HTML5. The content creation is where they make the money, after all.
Yes Flash with it's weekly or sometimes daily security vulnerabilities was bad, but HTML5 is not "better".
It is design by giant corporations (DRM inside your CPU, inside your browser, etc.) FOR other corporations. It is not designed for people, for freedom. It is regulatory capture by Silicon Valley "techbros" of... Internet's standardizing bodies instead of government regulating agencies.
Hence when I recently used the browser (designed by an advertising company BTW), websites just freely play annoying sounds.
This is not "better". It's worse IMHO, since at least with Flash you had click-to-play.
/Rant