No! is the right answer, as you don't have to be one or the other you can be both. I'm reading a lot of fear related blog posts as of late around Silverlight and wonder as to how this got to the state it's in?

You don't have to be so narrow minded about it, as FLASH and AJAX co-exist, why can't Silverlight? Laszlo and Flex co-exist? anyway, I think if you ignore the "Flash Killer" speeches and opinions and instead keep this in mind, you'll be fine going forward!

Keep moving forward.