The "buy once update forever" business model of the Mac App Store is the real financial burden, not the ~30% tariff. Dev houses need some residual income, esp in the relatively small Mac market.
My guess is this could be driving the divide between semantic and vanity versions. If a developer has a big new update that's fully backward compatible they'll still be tempted to bump the major version of it means they can justify selling it separately. (Assuming stores allow names with a numeric bump.)