...but why do patients care about the diagnosis on record?

Most people don't. Most people don't care about much of anything that's written about them in their chart and won't bother to read it. Which is why I think most of the concerns in this thread are a bit excessive.

I'm not based in the US, but my understanding is most countries have always allowed access to patient records and most don't do it unless they have a particular need.

So really, not much has changed. The small amount of patients who care to read just to see what is written about them already could do so. My clinic has had a couple patients access their records who were a bit unhappy about stuff that was written about them, but mostly just because they didn't like stuff along the lines of "Lengthy discussion with the patient about X,Y,Z. They still express some hesitancy about A,B,C despite the discussion."

As long as the documentation is done in a respectful and truthful manner, complaints will be minimal and probably come to very little. It's a good opportunity to remind providers to be careful about writing lots of stuff (beyond vitals, labs, etc.) that wasn't discussed with the patient, which isn't the best practice anyways.