No, it absolutely wasn't, as can testify anyone who actually had to work with it: https://www.cnet.com/tech/services-and-software/the-death-of-adobes-flash-is-lingering-not-sudden/

There are lots of good reasons to get rid of Flash. Browser makers say it's a top sore spot for security, performance and shorter battery life.

https://tedium.co/2021/01/01/adobe-flash-demise-history/

Usability means a few things in this context—simplicity, ease of use, convention, and accessibility. Flash was none of those things. It took the blank-canvas approach to creativity—which was great for the artists and illustrators that originally made up its target audience, but morphed into numerous other forms that it wasn’t necessarily designed for. It fell into overuse and quickly became abused by others.

Not really. To have fresh dynamic content having to install a third party plugin is a bad take. Web development was stagnating due to IE's market dominance.