According to Miyazaki, "If the Souls could be recovered anytime, there would be no suspense or sense of accomplishment." He added that he wants players to feel like, "I did it! I made it to my bloodstain!" Of course, Miyazaki fails to mention the sense of utter, despairing failure one feels when they don't make it back to their bloodstain (a frequent occurrence).
Regarding the possibility of a sequel, Miyazaki said he couldn't give a direct answer, but noted that the team learned a great deal on Demon's Souls and there were several things that didn't make it into the game. Given the opportunity, Miyazaki said he would "welcome the chance to create an even better game in the same style." Gluttons for punishment that we are, we'll take it.