On the one hand you have Flash, which has been around for ages and you can't just take it out without breaking a lot of the web. If someone found an open way to render Flash (like if Flash Gordon somehow got good enough to do it), I guarantee you that Google would remove Flash. They would much prefer this to having to bend over backwards to render it in a different process and blah blah blah. The only reason Apple was able to remove Flash was because they entered the mobile landscape, where having a browser that couldn't render Flash still rendered more of the "real web" than most competing mobile browsers at the time. However, you don't see them dropping support for it in Safari (and not including the plugin on some macs is not dropping support). If you couldn't run flash at all on a Mac, ever, it would be disastrous.
H.264 on the other hand is a brand new technology, there is still time to make the right move before we end up in the same position as we are with Flash today: everyone wanting it gone but it not being easy to remove it (similar to the GIF situation as well). On top of that Google controls the biggest video supplier of the web, thus they have the additional ability to make the transition less painful.