Most decent and successful devs will make just as much money with their next project as they did with their old, some actually earn quite a bit more. Given the millions of players and huge fanbase DC has by now, he'll probably always make a good amount of money - as long as he releases content. So it doesn't really matter whether he continues to work on SS or moves on to some other project.
The quality of the art is undeniable, even if its not your style of game.
Well, that's kinda the point why it took/takes this long. The art of game was actually quite badly done at first and only now that it's getting replaced, now that the game runs properly on mobile, etc. it became a really good game.
If DC had continued as things were at the start, we'd still be looking at...
..., there'd still be two frame animations, there'd no cut scenes and we'd still be having nothing else but "earn money and buy item x" quests.
So it's certainly true that the development isn't fast, but it's not like one would expect a lot more, given the way they work.