Maybe not
strictly insurmountable. But building your own massively distributed infrastructure capable of resisting DDOS attacks is expensive. Perhaps prohibitively so, depending on the size of your budget.
You could just as easily argue that since DNS blocks can technically be overcome by directing users to a specific IP address, or by using an alternate name server infrastructure like Namecoin, that DNS isn't critical.
In my opinion, the distinction is rather blurry.
