A lot of people are passionately involved in the "one way or the other" debate surrounding Flash and HTML5. Some people contest that Flash is, and will be for some years to come, the most popular and accessible form of bringing media to the masses. Others have put forward the argument that Flash is outdated and incompatible with some modern devices, and HTML5 offers many improvements over Adobe's antiquated format.
What's the right answer? It's simple, if you're a user: it doesn't matter what the answer is Whether you're watching YouTube, a client presentation or a tournament on Partypoker, it's not going to make a huge amount of difference to you as long as the media is brought to your fingertips with ease. A lot of people forget that not everyone is a web developer, so whether that embedded funny video is in Flash or HTML5 isn't going to make the slightest bit of difference to them.
That isn't to imply that the average user is stupid and only enjoys funny videos - that's a generalisation that's only too easy to wield, much to the unhappiness of those being stereotyped. Ironically, it's also this demographic that will ensure your sales keep going up, so not pandering to the PEBKAC, "read the manual" mindset is probably for the best.
If you're wondering which to choose, ask yourself this - which do you enjoy most? Do you like working in Flash, whether everything is familiar and easy to do, or do you find the challenge of a new format with new capabilities exciting? Either way, it's worth being knowledgeable about both, as a HTML5 switch-over is a reasonable possibility.
Looking into the future, there's a lot of turmoil for those who live designing for the web the way it is now. But at the end of it, some of you will adapt, and some of you will die stubbornly. HTML5 and Flash won't take your souls, and neither will those who debate with you. Learn as much as you can, and experiment - and good luck doing so. See you on the other side.