It takes away control from the user that no longer has full and relatively-easy control over the data flowing through their hardware. With Google controlling the browser, the web renderer, the HTTP protocols, the add-ons available in the browser and preventing data manipulation due to HTTPS, they have an encryption-protected pipe straight from their servers to the user's screen.

And it's all done under the guise and blessing of "privacy". It's really a bleak future for the web.

Sorry, I went on a bit of tangent. But to answer your question: HTTPS makes it incredibly difficult to introspect and alter content that is flowing through the web and your browser. Allowing a user (and his ISP if done correctly) to easily at the network level alter the content, one can do all sorts of magic that we haven't even begun to explore because it's effectively impossible.

The big usage of this would be ad-blocking and removal. At this point the two biggest ad-blocking mechanisms we easily have available are: DNS-blocking of ad servers, and add-ons/plugins that are allowing introspection of the data on the web pages visited. Both of those avenues are being attacked. Add-on APIs and capabilities are being neutered in little bits and pieces both on Firefox + Chrome. And DNS is being attacked with things such as DNS over HTTPS (again under the guise of privacy).

Not to mention that even SSL certificates that allow MITM for the user are being attacked by initiatives such as embedding SSL certificates into binaries, and certificate pinning (which luckily seems to have been abandoned).

We need FOSS/Stallman-level activism and wars against this stuff that is eating away at the rights we have over our own hardware. Whatever you call this issue, it should be right up there with "right to repair", "own our own data", "right to be forgotten", etc.

Edit, wrong acronym.