Empirically, some industries are approximately constant-cost, others are increasing- and decreasing-cost. OP mentioned certain factors pushing one way or the other, but ultimately the slope of the long-run supply curve of an industry is determined by which factors predominate, so we’d have to measure it to be sure. What is generally true, however, is that long-run supply is typically highly elastic, so cost doesn’t change much from marginal changes in demand.
Empirically, some industries are approximately constant-cost, others are increasing- and decreasing-cost. OP mentioned certain factors pushing one way or the other, but ultimately the slope of the long-run supply curve of an industry is determined by which factors predominate, so we’d have to measure it to be sure. What is generally true, however, is that long-run supply is typically highly elastic, so cost doesn’t change much from marginal changes in demand.