It would be great for more people to experience it, but unfortunately there are a few blocks in the way of this happening that make any chance of it occuring very, very small if not impossible.
The most important is the commercial success of Demon's Souls. More specifically, that there wasn't any "success" at all. It was a sales flop. Like Psychonauts, it was a brilliant game that virtually everyone who played it loved, but it didn't recieve wide enough attention and hype or advertising to actually sell well. If FROM tried to pitch this port to anyone, the response would likely be something like "we aren't going to dump loads of money into a port for a game that didn't even sell well to begin with."
Second is the actual process of making the port. The reason a PC port was doable was because the 360 shares very similar hardware to a PC, so very little programming shifts or compatability issues had to be dealt with. Making a port for the 360 version though was basically having to reprogram the game from scratch to match new hardware settings. This wasn't originally commercially viable for Demon's Souls at its release because the 360 itself has abyssmal sales in Japan and they weren't sure how the international audience would take to the game. The reason we got one for Dark Souls was because FROM realized it's international audience, read as; the places the 360 did sell well, would make it a worthwhile investment into extra costs to code a 360 version.
Lastly is the age of the game. On top of the bad sales and extra costs of porting, it's an old game. Again, back to the imaginary investors, but they would probably add in to their response something like "Well it didn't sell well the first time, why would it sell well this time?" Those who played it before wouldn't likely want to repurchase it, and several of the prospective customer base would be just as content sticking with Dark Souls, with the only driving force behind the older game being curiosity as to "what was Dark Souls like before?" They wouldn't want to buy an outdated version of their favorite game (Dark Souls) when they could just play Dark Souls.
Long story short, Demon's Souls was a commercial flop that was created years ago, and wouldn't be worth a company's time or money to make a port for another system.
This is unfortunate indeed, since Dark Souls did in fact bring over many fans to the series through the 360, and they have every right to experience such a wonderful game. However, short of the series exploding in popularity (despite how much more attention Dark Souls drew to it, it's still a rather under-the-radar game series) and gaining a collector's edition made for digital distribution in a few years, I doubt it will happen anytime soon. It will have to recieve similar treatment to Banjo-Kazooie, where the nostalgia factor of old-school Souls Series fans combined with the lessened costs of a purely digital title on the XBLA or PSN evens out to make it a commercially profitable idea.