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?