Putting those worst case scenarios aside for a moment, the no back door policy pushed by Cook is so that the phone isn't made less secure for hackers to get in. Yeah, it protects privacy as well, but I don't think it's a good idea making technology easier to hack in the rare chance that one of those scenarios above happens. I'm more afraid of hackers than I am of those scenarios above: those bad things happen, but hackers happen more often.
This. Bad things sometimes happen. It's very rare that encryption would completely prevent law enforcement from doing their jobs. They were able to investigate crimes fine before smart phones. Use other tactics.
On the flip side, an insecure encryption algorithm is a disaster waiting to happen. With all of the data breaches in the last few years (Target, TJX, Home Depot, OPM, Sony, Anthem, the list* goes on and on and on and on), only a fool would suggest that weakening cybersecurity would be a good idea. We need to be doing everything we can to harden encryption and improve cybersecurity, not weaken it. Thousands of data breaches already happen daily. We don't need to make it even easier.
* https://www.privacyrights.org/data-breach - see here for list.
Go ask Wall Street how they would feel about a known security flaw being mandated in all of their digital financial infrastructure.