But as long as mining is marginally (i.e. BTC income larger than electricity costs) profitable, people will join mining and difficulty will increase (that's how arbitrage works).
Exactly!
I don't think we will see difficulty increases of 20% per 2106 blocks (as mrjones suggests) but 10-15% is very real in the nearest future.
And even with that very conservative estimate of a 10% change per two weeks, difficulty increases by a factor of 12 (1.10
26 = 11.92) over the course of a year. By the end of that year, a rig that generated 50 BTC per week initially becomes one that takes an entire three months to generate 50 BTC!