I didn't downvote it because it was a plausible-sounding suggestion which led to a clarifying discussion, but my guess is that some downvoted it for giving a somewhat irrelevant/misleading answer. As Adele_L explains, heat loss pertains to the Second Law, while the proposed setup has the more fundamental problem of violating the First Law and even assuming away heat loss does not work because of gravitational redshift, as Plasmon explains.