I guess it all comes down to the value of Bitcoin itself, which is broken down into its constituents. For example:
- price of mining equipment
- price of energy
- value of the ideology

If you can get all the items for less than market price, then mining Bitcoin is worth it. I expect once we get to a certain block reward, the value of the ideology will need to go up 

 It’s possible that the difficulty adjustment will always make it worthwhile for tons of people to mine