You are probably right. What will happen is that ad-blocker people will indirectly kill accessibility. That would make a lot of sense in this world. Its a reoccuring pattern. Spam killed a part of accessibility indirectly via CAPTCHA. And "it is my god-given right to block ads of free services I use" people will indirectly finally kill accessibility for good, now that we have <canvas>.
However ads still need to be delivered over the net so there is still some way to block them (without resorting to router/firewall level blocking).