The difficulty does not NEED to be ever increasing. In fact there are periods of time where the difficulty is pretty stable, such as mid-2011 to early 2013, or the first half of 2015: <a href="http:&#x2F;&#x2F;bitcoin.sipa.be&#x2F;speed-ever.png" rel="nofollow">http:&#x2F;&#x2F;bitcoin.sipa.be&#x2F;speed-ever.png</a>