This would hopefully align incentives: we understand that ads are necessary for the web, but ad makers have to work on efficiency to increase revenue. This would theoretically be like "AMP for ads". If you use too much memory, your ads drop because you keep crashing on the page (just the ad of course).
If you wanted to be perverse, you could have a limit on TOTAL memory all ads could use on a page, so you end up creating group pressure on any individual that's not playing nice (they might start demanding not to be served on the same page as those ads etc). But maybe this is too much. Just limiting the total amount of "memory usage" is a good start.
The important thing here is that it hopefully leads to the world we actually want: REASONABLE ads that don't require crazy measures that end up hurting the sites we love. Browsers COULD ship this by default without hurting the ecosystem.