That is a pretty poor argument. There has never been any guarantee that ads will be rendered, and users rapidly learn to ignore areas of the screen where ads typically appear (I have seen user studies to this effect). "Reader mode" will basically prevent ads from being rendered, unless the ads are purely text and are inlined with the main text of the page. Calling ad blockers "theft" is as ridiculous as calling ads themselves "theft" -- after all, the ad companies are using a large fraction of the bandwidth users paid for, and the CPU cycles and electricity on their computers, without having been asked to do so and without having asked permission.

If websites really find that ad blockers are killing their revenue, they will switch to a paywall model, and maybe micropayments will finally happen (or perhaps a service that allows users to subscribe to large numbers of websites at a time). Whether or not that is a good thing depends on who you ask.