Except the downsides are mostly for the users (extensions stop working) while the upsides are mostly for the browser developers. (Better ability to refactor and update)
The upsides are for users as well: better performance, less memory usage, better ability to implement web standards. Browser developers aren't just breaking extensions for the fun of it, but because they're in the way of actually producing a better product; the goal is to produce said better product.
The upsides are just somewhat more diffuse than the downsides: the former apply to all users, while the latter only to those who have extensions that stop working.
