We still use plugins. In fact you most likely have one installed right now for video encoding. JavaScript not being a plugin is the reason we only have two major browser cores. Chromium and gecko. JavaScript prevents new browsers from entering the ecosystem due to how hard it is to implement unlike how easy it would have been as a plugin.
Flash had vulnerabilities because of neglect from adobe. The core design of flash and its earlier stages made by Macromedia were great. It had a sandboxes environment, and later it even was integrated into a browser sandbox just like JavaScript, eliminating most vulnerabilities.
Flash was very limited in the malicious code it could run, as opposed to JavaScript which can automatically redirect you to malicious websites, install tracking cookies, access the browser canvas to install tracking pixels, freeze your entire browser, take control of your cursor, look at your entire clipboard history, collect enough information about you to competely identify and track your footprint over the entire internet.
Flash couldn’t access your clipboard or files unless you clicked allow every time, couldn’t access anything outside of its little window, and if it froze, the browser was mostly unaffected, and flash had almost no ability to collect any data about your browser.