I'll use ETH as an example. I'm operating under the assumption that it's popularity (and hence market cap) is bound to rise faster than its difficulty. While mining rewards may dwindle as difficulty rises, the value of mined coins rises faster. Of course, we have no way to know for sure, but is my reasoning at least based on good logic?
This is all speculation. But consider this, if you are 100% sure the price will rise, then why not just buy the coin instead of going through the headache of mining.
But like I said before. Rising difficulty doesn't mean rising price. Just look at Bitcoins price and difficulty during 2014-2015.