"We don't know how to build it" -- if "we" is humans on planet Earth that's a large set and makes negatives hard to prove. There could be a yak herder in outer Mongolia right now sitting in a yurt with their laptop, coding the shared ledger solution, and you wouldn't know about it.
The "if/when it exists, the news will reach me" attitude requires being well plugged into the global conversation, and one that bitcoin "shutting out the blasphemy" bigots can't afford.