It shows a message which wastes valuable screen estate, especially on low resolution terminals, containing a message I have to read every single time because the keys are not in muscle memory, and never will because the bindings are stupid.
On systems I have control over the reaction to nano popping up is exiting, removing it, making sure the package system blocks reinstallation attempts, and go back to what I was initially doing in a sane editor.
My man, most of us aren't connecting to our mainframes on VT20s these days. Even on my phone screen the three extra lines nano takes over vi aren't a problem.
Also if you have the time to go through all that you have the time to learn ctrl+x.
Ctrl-X in Nano is arguably more nonsensical, considering that vi was made in an era long (decades) before many of the conventions we know today came about. They were figuring it out in real time. And the criterium here is much simpler: it must be available on all keyboards so no fancy keys. That's all.
On the other hand, when nano decided to use Ctrl+X for eXit, Apples Ctrl+X/C/V had already been brought over to Windows and Apple, and was also the de facto way for most Linux apps to handle these inputs although I do think it came before any "official" efforts to standardize these shortcuts in desktop environments.
It doesn't have to be X, just the fact that it uses modifier keys is enough. It could be Q or anything else, just please, for the love of god, we live in the 21st century now, all keyboards have modifier keys, please, add modifier keys shortcuts as well.
I don't do that much search and replace in any terminal based text editor to actually use that on a regular basis. If I need edits like that, I use a GUI text editor.
As long as I get to use modifier keys, almost anything is fine with me. We don't live in the 70s, that was 50 years ago. If backwards compatibility is what they're after, I'm sorry but I think they overdid it. Plus, you can just add them, the defaults don't need to be changed.