"You're taking something subjective (controllers being simpler and easier to use) and using that to try and negate something objective (mice are better suited for playing FPS games"
You seem to be confusing the term "better" with more accurate.
This is your problem.
No one's arguing that m/kb isn't more accurate than a controller. Of course it is. Of course you have more keys on a keyboard that can be assigned to many different actions than you have buttons on a controller.
There's no debate there.
Your problem is that you're using the terms "better" and "better suited", as though everyone who uses a m/kb will have a more enjoyable experience. That isn't the case.
You even prove this point yourself, in your own words, when you say:
"Literally anyone can pick up a console FPS and do well. You can run around like a headless chicken and luck your way up the scoreboard.
It takes a lot of skill to be consistently good at a PC FPS however, especially the hardcore ones that have a steep learning curve."
This utterly destroys your own claim that m/kb is "better".
Going back to the fact about sales of console FPSs vs PC FPSs, if one thing is clear, it's that most people simply don't want a "hardcore" experience.
They either don't have time, or they find it boring, or like you say, they just find it too difficult.
For every single one of these people, a m/kb is not "better" than the relative ease and simplicity that comes from using a controller. They want their game to be fun, not multitasking nightmare.
They simply don't have the skill, and probably wouldn't even play FPSs at all if they didn't have a controller that made things more simple for them.
So for you to tell all of these people that they simply don't know what they should like or not like, and that they don't know what's better and what's worse, is simply arrogance.