I loved Dark Souls, and am currently playing through Dark Souls 2. HOWEVER I have never played Demon's Souls much beyond the point of a very quick curiosity when it came out. I died, got scared, and never played again.

Now I want to go back and play it, but is it popular enough to play co-op/summon in others to help? I've played Souls games solo for the most part, but if a boss starts to give me trouble I've liked having the option of summoning help right outside a gate. Is this too old for it to be feasible? I don't want to wait for half an hour for a summon sign to show up.

Chances are there will still be some people there, though I can't say much beyond that. I suppose you can assume that most players who invade you will be min-maxing and be able to kill you easily.

When I was playing Dark Souls on the PC prior to Dark Souls 2, outside of the more popular areas, summon signs weren't entirely common. The Manus boss fight for instance had NO summon signs, so I had to solo that damn thing, god I hated it. I cab't imagine it being much better for Demon's Souls.

I played Demon's Souls not that long ago and it still seemed to be active enough. Summon signs to summon. Invaders invading. Etc.

Most of the bosses are on the easier side in Demon's Souls though, to be honest.

I really can't say, but Demon Souls probably has the easiest bosses of all the Souls games so you probably wouldn't need any help.