Good article Stephen!
The master-key idea in almost every case backfires, they tend to be leaked/sold very soon afterwards and are soon available on the open market.
Why is that a bad idea?
John Oliver summed it up pretty well:

So for a very uncertain gain in security against terrorism, she is willing to create an almost certain (and vastly bigger) loss of security against cyber criminals targeting economic targets.
Aside from the problems with security of dissidents, freedom of speech etc...

Fact is the Paris attacks were coordinated with unencrypted SMS text's, they did not even need crypto.

Also pertaining to cryptocurrency, there is no reason they need to abuse that to get funding.
Interestingly the Islamic world already had a very interesting anonymous and huge payment network which existed since the 8th century between Arabic and Muslim traders alongside the Silk Road .
This is one of the things that terrorists have been abusing to transfer funds.

Check out Hawala it is actually a pretty cool concept and worked for centuries!
From Wikipedia
"In the most basic variant of the hawala system, money is transferred via a network of hawala brokers, or hawaladars. It is the transfer of money without actually moving it. In fact, a successful definition of the hawala system that is used is "money transfer without money movement".

https://en.wikipedia.org/wiki/Hawala