Ignoring the miners long term incentives to benefit the ecosystem and assuming short term selfish behavior only, the miners will be incentivized by more tx fees when the capacity more than doubles. When margins are extremely tight these incentives are more than enough to encourage the right behavior.
You seem to be assuming that the "fee market" will set in and space in blocks will be a scarce resource. But, if a sizable fraction of the traffic adopts SW, the fee market will be delayed by another 2 years at least. Large junk traffic generators like SatoshiDice can avoid paying higher fees by using the SW format, while miners and full non-mining nodes will see their bandwidth demands increase beyond the 1 MB mark, just as if the block size limit had been raised to 4 MB right now.