I hereby crown you Kings of Tower Defence.
Seriously though, the reason your game keeps selling is because it is phenomenal.
This means viral sales, from fans buying it for their friends (did that), bugging them to buy the Humble Bundle it is in (same) and then actually motivating them to play the game (you wouldn't believe the backlogs people have).
Another clever step you took in this direction is that you gave DQ2 pre-orders (of course I did) a free copy of DQ1 to delight their friends with.
Also, game updates not adding anything substantial? How about the New Game+ feature that pretty much gives players the end-game content they have so been craving that is the same size as the rest of the game?
None of the things you said were wrong, but a lot of it was more humble than I feel it had need to be.
Because Defender's Quest really is something special.