UA manages inventory to maximize yield, not to placate GS members. If there was no GS level, I doubt it would have much of an impact on inventory management. Ultimately, I'm fairly certain having fewer people around who purchase the last minute high fare/full fare tickets is a bad thing, not a good thing. I fail to find your argument compelling.
Of course, I absolutely agree with your first point. There is an opportunity cost of hanging on to inventory, so shouldn't the possible buyers of those seats be saddled with the burden of the seats going out empty or on the cheap?