It's not about being defeatist, atleast not for me. It's about what is considered good enough.

Sure, locking down the OS in this way is more secure, but it's also very restrictive and personally I don't think the added security justifies this. Lock picks do exist, but I am still entirely content with a single lock on my front door. I do not need an extra biometric sensor or camera or security representative standing outside my door to check id's of people passing by in order to consider myself reasonably safe.

Maybe this is cultural/geographical, but I've yet to hear of anyone who lost access to their mail or had unauthorized access to their bank account as a result of malware. I'm sure you can find examples, but I do not consider this an attack vector that is prevalent enough to warrant requiring signed apps or preventing manual installation.