Christian theology teaches that one should desire neither poverty nor riches, and that the rich have a responsibility to scale back their own consumption to leave some for the poor, so I think "minimalism is pro-social" is accurate from the standpoint of Christian ethics.

As to the negative effects on producers, one should remember that everyone is both a consumer and a producer, and that "producer surplus" so-to-speak isn't well defined apart from the consumption that this surplus buys for the producer. Thus, if the producer has adopted a minimalist lifestyle in their consumption patterns, it's not obvious to me that a minimalist society would make producers worse off.

Ultimately, though, this discussion highlights one of the weaknesses of traditional economic welfare analysis. Without a clear ethical foundation from which to make judgments of "good" or "bad", we're just lost at sea like a ship without a compass. We gravitate towards "more GDP", which essentially means "more stuff", not because that's morally right, but because we can't agree on a more comprehensive ethical measure.

(To be fair, it's also because economists are trying to be as "scientific" as possible and to import as few ethical assumptions as possible. "More stuff" is always "better" in a weak sense because you can always choose not to consume the stuff that's there. Still, given that producing "stuff" results in the usage of input resources and given that disposal is not actually free, maximizing "stuff" isn't really a great metric to work with.)