I suppose that depends entirely on the amount of Shardbearers in one world. Fifteen Shardbearers in one world versus, let's say, three. One of the fifteen loses their shard and the damage isn't quite so bad, as there are fourteen more connections to that world to mitigate the damage. However, if a Shardbearer from the trio dies, then the impact would be much larger. As long as there's one Shardbearer from that world, it'll continue to live. At least, that's what I've been able to conclude.

[Joshua hums in thought.]

A very complicated question. Shards anchor us to our worlds, so you can say that we have a somewhat symbiotic relationship with our shards and our world. My guess is that the reason they left is because their connection was severed in a significantly different way. These people aren't dead so much as they are lost between the worlds. What that means precisely, I don't know and might never know for sure. Given that we can come back either with or without our memories of this place, it's impossible to know what truly happened.

On the other hand, if this severance is a death, then the question is why do some people come back? My only guess to answer this particular question is that the parallel worlds theory holds water. Let's say you disappeared from this world, Leo. Perhaps two months later, a Leo Valdez returns with no memory of the Drabwurld, but with the exact same memories of the events that happened to you in your home world. However, this version of Leo likes playing Yahtzee, even though you hated it. A very small difference and most will never pick up on these differences with how closely similar the two of you are, but he is still a different version of you in a very strictly technical sense.