There is no reason that over time, they WON'T keep scaling this game for both PC and later, Playstation 4. (I believe Yoshi P even said/hinted during the last live letter that they do plan on upgrading the game overtime) However, currently, the market is better supported using the PS3 and PC model. Not everyone has 'awesome' rigs. Go check out some of the benchmark scores posts. There are people playing on all sorts of machines due to what they can afford, how much graphics matter to them personally, space, etc.
Also, by marketing to the PS3 crowd, you get to market to an otherwise untapped MMO playerbase. PS3's are more affordable than gaming computers, and not only that, but as I don't believe there are PS3 MMO's, or at least there would be very few, the competition to keep the players attention on FFXIV and not another MMO is lessened. FFXI was a gateway MMO for many people, FFXIV might serve the same function.
Part of the problem 1.0 had in even getting an audience back at initial release was that it basically required a new, fairly expensive/top of the line computer...or that the user understood how to build a gaming computer. It's great that so many of you know how to build solid rigs or have the money to buy them, but not every gamer (most in fact) can afford such machinery...or to upgrade their gaming consoles/computers every 2 years. Offering the PS3 as an option to play as well as lessening the system requirements* to play on a PC help open up the game to a wider playerbase.
It's actually a good thing they are doing their best to release this game on both PS3 and PC...it is also great that they learned to lessen* the PC requirements for the game. It will mean a much larger player base which means more people to play with for us but also more subs for them to continue to develop and update the game. In 4-6+ years, if the PS3 starts limiting what they can do development wise and if the household ownership/cost of owning a PS4 is reasonable, I'm sure they will try to upgrade to a PS4 version of the game....unlocking more longetivity for PC users if needed. Although seriously, the game seems to run fairly smooth and looks great in the current version so really, what's the worth in worrying about what it could be like in 5+ years?
*I don't think they lessened the system requirements for PC as much as 2 years time has passed and people have newer machines/upgraded machines in general now. I have not compared the original system requirements from 1.0 release to the recommended ARR ones. I just know ARR will run better on the same computer I used for 1.0.