Flash isn't terrible. Flash as a browser plugin was terrible and insecure indeed. Flash was no more terrible than Java or .net on the desktop as a multimedia app platform. In fact it made it easy not only to create interactive "experiences", games, but also artist authoring tools. Alchemy allowed to incorporate C/++ code into Flash, and Flash also had its own shader editor for effects. Adobe threw the baby with the bath water.
Actually it has a good use case for videos and animation. Flash animation uses vector graphics, saving huge amounts of space for basic animations and allowing the animation to scale to any screen size without degrading resolution. Part of the reason Flash took off on the web was due to small size (useful when network throughput was more limited).