[admax88q]

If it became that common it would quickly cease to be that effective. Spearfishing works because it's rare, so it doesn't automatically set off your bullshit detector. Most people don't fall for "cheap vi@gra" emails anymore.

Social engineering in general is effective because it's rare enough that people don't feel the need to develop policies and strategies for preventing it.

[SaltyLemonZest]

Sure, but there's no reason to expect the required strategies to be non-disruptive. It's now impossible for anyone not on my contact list to call me, because I won't pick up or listen to their messages - it'd be a tragedy if email became similarly locked down.