Will the increasing difficulty in Bitcoin mining push miners to a different currency? I've been reading all of the "Is mining still profitable?" questions, and the consensus appears to be the following: Yes, difficulty is increasing, but profitability is uncertain due to the speculative nature of mining. 

What incentive is there to keep miners on the Bitcoin blockchain? If their ROI is positive, but marginal return is decreasing, why not switch to mining a new currency?

I'm new to Bitcoin, I apologize if the question is unclear or illogical.
It appears that miners consider Bitcoin to provide the largest ROI.

There are dozens of other coins that use SHA-256D as their hashing algorithm, if it were more profitable to mine any of those, miners would switch. 

However, when the hash rate increases in the other coin, the revenue per hash rate decreases. Meanwhile, the decrease of hash rate in Bitcoin increases the revenue per hash rate there. Thus, anyone leaving Bitcoin makes it more profitable to mine Bitcoin and vice versa. Thus, the system self-balances.

AFAIK, most altcoins' exchange rates have been in a downward trajectory which probably doesn't help to entice mining power.