First of all yes ratings can stop a game from selling well on certain consoles. Even the market games have been catered too have resulted on failures on the Wii. There's a list of companies that openly announced that they wouldn't support the Wii with M rated games for specifically this reason.
Sega said it after the double flops of House of the Dead Overkill and MadWorld
Capcom is lowering support after the flop sales of RE
arkside Chronicles as well as the "core" orientated Zack and Wiki flopping.
EA slowed down support for the Wii due to low sales and they don't even make a lot of "M" Rated games.
And I looked up 999's sales. They were barely a success at 170,000 after 1.25 years.
GTA Chinatown Wars sold 1.3 million of the DS only because they slashed the price %50 after the abysmal first two weeks it had. Rockstar made no money off of GTA:CW.
And keep in mind that the guys who made 999 (Aksys) are a pretty small company so 170,000 is a lot to them but nowhere near impressive. 170,000 people equates to under 1 percent of all DS users. Not the best example to use in this arguement.
Demon's Souls selling over 1 million units on just the PS3 based on purely word of mouth is impressive.
And there are clearly more children buying Phoenix Wright than GTA. There've been sequels and spin-offs of Phoenix Wright. Seeing as how kids can't buy GTA itself legally, and GTA has been blasted in the news for over a decade due to non-child friendly content :S