Once you have a sustainable process, these last holdout areas like dredging (where you can't use batteries or wires) can have relatively terrible EROI as long as they're a small minority of the economy. All aviation and shipping combined is about 6% of final energy, and with major redesigns for shipping it could be as low as 4%. So you'd trade a much better EROI for 96% of the economy for a terrible one that only affects 4%.