Perhaps there was a memory leak in Unidentified Flying Assholes or the endless line of punch-a-celeb games or the thousands of stick fight productions and so on, but no one cared and enjoyed them immensely anyway. You could do something cool without ever learning about things like memory leaks or vulnerabilities in the underlying platform.
Some of that did, at least for how that creative work was almost exclusively delivered to the world. Those bugs were not just excessive resource usage and instability, they were incredibly often exploitable security flaws that were regularly weaponized against a huge swath of internet users. The ubiquity of the Flash browser plugin was simultaneously one of the greatest strengths of Flash as a creative platform and one of the greatest risks to the average person browsing the web in the 2000s.
The plugin needed to die. Unfortunately the Flash community was so firmly built around the web plugin as their distribution method of choice (presumably because many of us were browsing animations and playing games at work/school where we couldn't necessarily download and run arbitrary .exes) that the plugin was more or less a diseased conjoined twin, and when it died the community didn't have long left.
Compare this to Java where the death of the browser plugin caused a number of badly designed banking sites to have to be redesigned in a less stupid (but quite often still very stupid) way but the community as a whole continued on without huge disruption. The browser plugin was just one of many places Java existed, it wasn't the dominant focus of the community.
- Vector drawing and rendering for extremely fast performance and file size
- Visual authoring tool that invited creative, non-technical people to the party
- Deep support for managing state changes over time
- Gradual ramp of complexity that balanced ease of entry without overly constraining expertise
Were most Flash apps slow and buggy? Yes
Did Flex have tons of bloat and memory leaks? Yes
Did Flash create a cambrian explosion of creative and fun projects that inspired a generation of young people? Yes
Now, the code people wrote was CPU hogs, because lots of non coders were writing code and they would do anything to make it work. The Flash runtime was not causing the Punch the Monkey and to peg your CPU, it was because the punch the monkey ad was fucking awful code.
All those Flash programmer went on to write the first wave of HTML5 stuff which, shock horror, where vastly CPU inefficient.