Since email deletion policies aren't obstruction until you're told to suspend them, there's no way an encryption policy is obstruction.
> to be a literal get-out-of-jail-free card for crypto nuts
Pff. It's not like simply not writing things down is a get out of jail free card.
So technicalities like you're invoking (is refusing to do something "obstruction" or not?) need to be balanced against technicalities on the other side (is providing a decryption key "testimony"?). And when courts have had to make decisions like this they've almost always done it by splitting the difference in some way instead of finding an absolute interpretation on one side or the other.
I'm not saying I disagree with you in principle, I'm saying that very reasonable courts might not. This isn't a cut and dry argument, at all.
This is precisely why "Obstruction of Justice" is a criminal offense with severe penalties available. The question then becomes: is deliberately encrypting and refusing to provide access to those books obstruction?
I know the folks here would argue otherwise, but IMHO it's not at all a clear argument legally. The original reasoning behind the fifth amendment was that without that protection the government would be tempted to use coercive tactics to induce a false confession. It's designed to prevent the torture of accused witches, not to be a literal get-out-of-jail-free card for crypto nuts.