I seem to be doing a lot of negative posts against indie releases on GOG, which makes me uncomfortable, but Niels Bauer was another of those holdouts that insisted that late nineties/early aughts era game mechanics and graphics should still be charged a very high price.

If you want you can pay $30 on his website, and just another $5 simultaneously nets you the archive versions I through IV, which are pretty much the same game with fewer incremental features. Or you can pay $15 for all of them if you're not buying V -- what a deal! ;-)

Even at the $9.99 "discount" ("discount" being directly quoted from Niels' Sep 15 comment on the Steam Greenlight page), this is vaguely the same quality as a typical $5-$6 mobile game. So when it goes on sale at the 60% discount price it'll probably be worth picking up, as you *do* get a decent amount of gameplay for your buck, where it would then rank among the "Good Old" like GOG used to stand for.

I do wonder why the price point wasn't set at the $5.99 level, which would have significantly boosted sales...

It seems to me like you answered your own question. The creator doesn't want to go any lower.

Aside from the fact that it wasn't really a question, it was more to point out that I'm having difficulty understanding why he's charging even $10 for the product. ;-)

Probably because the game isn't that bad and he wants to earn a living.

Other than that maybe he wants to sell his games to elitist rather than cheapskates.
Selling 500 copies for 10 Bucks or selling 1000 copies for 5 bucks doesn't make a difference, does it?
While the first solution offers the possibility to drop the price to 5 bucks later in order to get 1000 copies sold at an average of $7.50.
But why don't ask you him personally? He even included a photo of himself in the credits (something I found a little odd) So some "fan-mail" asking questions wouldn't hurt...

[edit]

As far as I know he actually has an account here, so you could just PM him and ask him directly.

It is a fun enough game for the price. Plenty stuff I would want improved about it for sure, but its a bit harsh saying it is the same quality as a typical low budget mobile game.