I think that if the game on this site drops the price from 20 to 10, that they'll surely get a few more purchases, but most likely not a lot.
I'm completely unaware on why or how they won't drop the price, be it they don't have enough money and sell it full price (I think not), or are just too busy with their new game development, Derek on 360 Spelunky, Alec on Marian and whatnot more.
As to your 3 points: First the fact that they got a lot of money doesn't mean you would lower the price of your game immediately (I know it ain't immediately after this many years, but my point stands).
Second, yes it's normal for most mass produced games to lower their pricing, but I believe Aquaria never went above the 20, so I'd say just let it. I enjoyed Aquaria a whole lot more than a crappy game called TimeShift whereas I paid 50€ for, now it's either 10€ or 5€, if I'd played the game before buying it I would've known it's not even worth that much now.
Third, lowering prices is indeed not saying that your game sucks and thus it should be cheaper, though again, Timeshift; games like that really aren't worth a lot, but besides that it does mean it's not selling as good anymore. Second to that, the Humble Bundle, anyone who hasn't bought that... well I'm not even sure who wouldn't have, but still, anyone letting that chance slide might as well pay the full price.
And I'm a frugal myself, I've spend so much money in the past on crappy single player games that I either got bored of real quickly or were finished within a day that I seriously don't buy them anymore. I usually just forget about them or if they look interesting, I play the demo or pirate, see how good they are from the first few minutes of gameplay and if I like it, I buy it. I'm fairly sure I started doing this right after having experienced Aquaria as well.