I agree with most of what you are saying except for two parts.The number of people who put off getting their computer repaired for something that doesn't render the computer inoperable is probably extremely high.
In your example of a cracked phone screen, most people don’t get those fixed because they already know it was their fault and they’d have to pay for it to get fixed. They also know that getting it fixed could potentially be expensive.
Second, the problem is with the keyboard, that the majority of the key failures actually do in essence make the computer inoperable. A failed key means you can’t type the letter b, or e, or whatever letter it is without an external keyboard. Or if it was duplicate keystrokes being received, constantly deleting the second letter is time consuming. This is something most people would take in.
Now, I do agree with @537635 point, that many people buy MacBooks for the cosmetic appeal and simply use their machine for video content or basic web-browsing. Those users indeed would likely never notice or ignore the keyboard issue. But for the large numbers of professionals and students, I don’t see them skipping on bringing the machine in to look at a failed key.
But like you said, we don’t have any true numbers and we never will. Ultimately we are all guessing. I have had my 2017 MBP for almost a year with no issues, but I know I am at risk. After 4 years I’ll likely upgrade to get something new that hopefully won’t have the keyboard issues.