What are the chances of someone coming up with a new and simple/out-of-the-box solution for the double-spending problem? The proof-of-work solution is simple but has many drawbacks (waste of time, energy and uncertainties about its security in the future).

The consensus solutions proposed by Ripple/Stellar are too complex, and with complexity also comes uncertainty/risk. (See Stellar catastrophic failure!)

What do you think are the chances of someone coming with a simpler solution, out of the box, something much more straightforward that no-one has thought about it?
The double-spending problem had been unsolved in decentralized protocols for decades. It seems unlikely that a second (different) solution would suddenly materialize.

It never was a problem in centralized setups, where the central entity would provide a referee function.