Clearly, HTML5's star is rising. But declaring that Flash should be retired in favor of HTML5 is a bit like saying that everyone should adopt electric cars. Although we can all agree it's a good idea conceptually, the infrastructure is just not there yet, particularly for online advertising. Digital advertisers need to make an informed choice about what type of media best serves the campaign.
There is no doubt that the increasing share of devices that don't do Flash is putting pressure on creative agencies to build ads using HTML5, and thus avoid building multiple versions to reach tablet users. With differences in screen size and ad delivery, the smartphone was just a warning shot. But for tablets, there is a real opportunity to share assets with the desktop-targeted version of the ad.
Adding to the pressure, Microsoft has joined Safari in banning plug-ins from Metro, the "tablet-version" of IE10 that will be included in Windows 8 shipping this year. With Microsoft and Apple both disavowing Flash (and all browser plug-ins, for that matter) in their future-facing operating systems, it is clear why agencies are viewing HTML5 as the future.
If you view tablet growth and future OS/browser support as the respective rock and hard place that will eventually squeeze Flash out of existence, then the ascension of HTML5 seems to be a given. But it's worth considering what we may be losing as advertisers looking to optimize consumer experiences with brands.
While the proprietary "black box" nature of Flash and browser plug-ins in general have been decried by web standards enthusiasts, there are many strengths inherent in the approach that have yet to be replicated with HTML5. Flash content, for example, is portable across browsers and can even be loaded inside other Flash content such as Flash video players. Embed it in a page, and you can be assured that the content in the envelope of the plug-in will not interact with the content outside it. The self-contained nature of the plug-in makes the content visually consistent wherever it's displayed, whereas HTML5 content is subject to differences in browser layout rules and typeface differences across platforms or browsers.
On the authoring side, Flash design and development tools are mature and full-featured. In addition to Flash itself, you have Flash Builder and a number of third-party, code-oriented IDE's like FDT, and Flash has well-developed integration with design software like Photoshop, Illustrator, and Fireworks. That integration enables designers to more easily go from a multi-layered design layout to interactive content. In addition, it's easier to work with scalable low-bandwidth vector content in Flash than any of the HTML5 tools currently available, and having the final output as a single optimized binary file (.swf), rather than a directory of scripts and assets, reduces the complexity of distributing your finished work.