That's not the fault of the underlying technology, if anything it will be EASIER for predatory adTech to display such things when the entire UI is rendered in an opaque <canvas>-equivalent. How do I get uBlock to block specific elements (like a video player) when there is no longer a discrete video element to block?

It isn't, but the sad state of affairs is that if it exist it will be abused.

... yes, and welcome to this thread which is about alternatives that tries to avoid that.