In general, arguments based on energy returns, technological advancements, and ecological limits are interesting in the close attention they pay to factors overlooked.  However, to the degree that they take attention away from what were (in my opinon, at any rate) purely political decisions, or the outcomes of fights between political and economic elites, they are a serious distraction.  In a way, they seem part and parcel of the same project that underlies much of economics, to remove a large part of human life from the realm of agency and choice, and reify it as a force of nature.

I agree that any approach can be used to distract from rather than illuminate a problem. But the political decisions, IMO, have to be viewed in the terms of How Institutions Think, as described by Mary Douglas. I have written on this subject in comments that TGB turned into a diary: Malleable social reality. One of the comments in that diary has a portion of Douglas' argument quoted. Christianity was an available option as a state religion in the time of Constantine, so he could make it and official religion by decree.  An effective alternate understanding and organization of Roman society in the Western Empire was not similarly available to my knowledge. That is a major reason to push alternative organizations of contemporary societies and economies into the public sphere today - so they are familiar and available to be considered in case the opportunity arises.

As the Dutch said while fighting the Spanish: "It is not necessary to have hope in order to persevere."