I guess making money is not reason enough.
And they stand to make a lot more money by remaining a platform holder that charges licensing fees on all software sold than they do by going third party.
Their first party offerings also stand to make a lot more money by not having to pay that licensing fee to another platform holder.
Sega didn't become a shell of their former self because they dropped out of the hardware business, they dropped out of the hardware business because of deeply rooted problems that caused them to loose touch with the market and eventually become a shell of their former self. It's not far from the direction Sony is heading now. I wouldn't want to see them go, but the industry would adapt, it always does.
During the Dreamcast days, their software was NOT the problem. They had quality and as far as I'm aware the software was selling fairly well. But in the end they couldn't compete with Sony, Nintendo, and an emerging Microsoft entering the market.
Also, Sony is nowhere near where Sega was. Sony's problems aren't with their gaming division, which has been profitable for quite some time, it's with their TV division which is bleeding the company. In terms of market presence, Sony has considerable marketshare globally w/ the PS3. The videogame division is one of the few bright spots in Sony's corporate arsenal right now.
And yes, the industry does adapt, but it's never the same otherwise. Sega had top-notch, unique content that has been sorely missed since the Dreamcast days. If Sony's studios were to similarly erode, that would be a huge blow to the industry and to gamers.