There was a time when Flash was one of the only reliable solutions for adding custom font faces to text without relying on manually created imagery. HTML5 supports media tags that allow to embed audio and video with almost all of the benefits of Flash video. Google has introduced a Flash free version of YouTube which sort of makes this HTML5 talk a bit premature. HTML5 support in browsers is still immature. IE8 does not yet offer video tag support. Most probably with the release of IE9, the HTML5 video will become default.
Let us consider the scenario of the rise and fall of Java. There was a time when Java was the popular technology for developing web design applications. But with the introduction of server side technologies like PHP and .NET, the use of Java became limited to specialized use cases. Although Java has not vanished and it still has some wonderful use cases, but it certainly is not the most popular technology it used to be during 2000.
Flash is quite effective and a simple, graphical tool when it comes to creating simple learning applications. Adobe may recognize that relative ease of use, with its feet firmly established in the industry, may be its biggest long term assets, more so than its use and adoption on the web today. Adobe is making use of several technologies that may take it beyond desktop web browsers with the upcoming CSS. The most interesting move is going to be the support for publishing iPhone apps.
It should be remembered that with web technology, the web developers can only move as the audience is willing to move. Approximately 20% of the users still browse with internet explorer6 which is a 9 year old technology. The HTML5 video is still in its infancy. Hence, it would be too far fetched to say that all the users will be on HTML5 aware browsers within, say, 5 years. This is something we can hope for, but certainly cannot be certain of. Therefore, it is pretty much doubtful whether Flash will disappear or will be replaced within 5 years. However, it cannot be denied that the influence of Flash is bound to wane away with time, and its adoption will diminish considerably.
As far as web developers and web development is concerned, no specific development technology is a safe “long term” bet in this rapidly changing field. If we talk about the situation after ten years from now, almost every engineering technology we use now will be antiquated.