In my opinion and after reading countless hours of discussions of devs arguing with each other is... we don't really know. But what seems more logical to me personally is the Gavin approach, aka, keep incrementing the size in increments of 8mb across the next 40 years or so. Technological advancement should cater for the space. Also, add in LN as well. We need both sides.
Other than Gavin's approach i believe we should consider Sidechains as a possible turnaround. I heard about the Blockcypher proposal and the Bitcoin Lightning Network proposal as well, wich presents off chains solutions to the scalability issue. I think this is also something to be considered!