[Arnavion]

>If so, what happens when mining reaches the point where it becomes economically worthless to everyone except a few electricity thieves?

The difficulty of mining is represented by a certain target number. The process of mining involves finding another number (the nonce) such that the hash of the transactions and that nonce together is less than the target. The smaller the target is, the less possible solutions exist, and the more nonces you'd have to guess before you find one that satisfies the condition.

This difficulty is self-adjusting based on how long it took to find the previous 2016 blocks (at an average of 1 block every 10 minutes this is 2 weeks). If the previous 2016 blocks were found faster than an average of 1 every 10 minutes, then the difficulty will be increased so that the next 2016 go closer to 1 every 10 minutes, and vice versa. (There is a clamp on the change of 25% if I remember correctly, so it won't change drastically.)

The assumption in the end is that these few electricity thieves will not have so much hashpower that they will be able to drive up the difficulty so high the rest of the non-thieves can get profits from their miners.