3 to 5 years! I expect all of my mechs to last a lifetime. My daily driver is cherry mx blues that have been clicking away for 11 years now, 6-10 hours of software engineering a day.
That calculation does not seem very realistic. I'd expect a mech keyboard to continue to function after 50 years of daily use, and a traditional membrane to probably wear out somewhere between 10 - 15 years of daily use.
I've gone through four or five Apple Keyboard (A1243) during the past decade. They break very easily, but I like the feel and layout.
With mechanical keyboards made for gaming (Razer and Corsair) I've had the caps start to fall apart and break after ~2 years of regular usage. The most frequently used switches also tend to start showing signs of wear and tear after a while.
That's why a lot of people into this hobby get pcb's where they don't have to solder in the switches and if ever a switch breaks they just pop in a replacement or pull the switch apart to see what's wrong. Similarly if a keycap `falls apart and breaks` although i don't quite know how this managed to happen for you
I have been using a Tex Beetle TKL Mechanical Keyboard for about 10 years and they no longer sell it which is a huge bummer. I'm sure if it breaks I'll be OK with another TLK keyboard out there on the market but I do love this one.