Yes HTML is necessary as compare to flash because html and CSS is main for web designing. Flash is a coding language and need a flash player to read flash code. Flash make possible to add animated image or content to website but it doesn't have an impact on ranking because search engine cannot read flash.
But both HTML5 and Flash are effective technologies, and since each one comes its own points of interest and disservices, I want to say that they're both victors and play very important role in web designing. It all relies on upon what you require them for.