As the amount of commerce on bitcoin increases, it becomes more desirable to reverse transactions, so there is indeed some ratio of "GDP to energy expenditure" that needs to be maintained for a sufficient level of network security.

So yeah it doesn't need constantly increasing energy expenditure as long as the amount of commerce using bitcoin stops increasing.

Indeed, and as miner reward continues to drop every four years, so will the budget to waste energy.

Eventually bitcoin will need to pay its own security through transaction fees.

Yep. That's what's expected to happen as the inflation rate drops to zero.