First of all, the QR code itself can be replaced without hacking. For example, if it's a sticker, someone can put their own sticker on top of the original one. If it's a photo posted on social media, someone can edit it in Photoshop to put their address. You get the idea.
But on software level, this task is not as trivial as replacing a clipboard, but still could be achieved, at least theoretically. The QR-code scanner app could be exploited to replace Bitcoin adresses with hacker's address, if it has such a sophisticated vulnerability.
You see? In summary, it then simply means that the security of our funds lies only with us and not on some gadgets. These gadgets, to start with, are the creation of man. Man can manipulate them. It takes a man to break any system or manipulate any computer or software.
I haven't received Bitcoin or any crypto through a QR before. I consider it a more complicated process than copying and pasting addresses. When it comes to financial issues, whether with Bitcoin or fiat, I think those involved should exercise caution and patience. Go through the details slowly. That's what I do. I'm never going to be in a haste. What for? Except it's an old address that I've saved up on my wallet, I take my time to run through new addresses meticulously.