If its possible to watch the video, then it's possible to watch the video without ads.
Worst case scenario: videos can be downloaded and adverts stripped from them. (If you can watch it, you can copy it.) Would you be prepared to trade, say, a 20 minute timeshift delay on your YouTube videos' initial publish time for no adverts? I would.
Which will just push people towards file sharing. If your DRM makes your service less convenient than copyright infringement, people will infringe copyright.
If companies start getting too draconian, the ad-blocking/circumvention/copying/sharing technologies will start getting smarter and harder to detect and circumvent. It is a battle that cannot be won.
I'd say the main obstacle in the short term is that as Google controls both client side (Chrome) for the majority and server-side can manipulate web standards to make ad-blocking harder, by exploiting their near-monopoly. They've already done this to an extent by modifying browser extension APIs. But people can just switch browsers. I've already done that on mobile. And if ChromeOS prevents it, I'll be erasing it and installing native Linux.