It always seems to me the people enthusiastiac about death possibility in these discussions aren’t the people doing the dying. Or aren’t the people that spent months working on a backstory For a promised year plus campaign.
As far as backstory goes, I do feel that most "backstory" should be developed from when you were levels 1-3. Knowing where your character comes from is cool and useful and such. If someone came to me with a paragraph or so with some highlights, a complication or motivation of some some sort, that's great! But, in my mind, the most interesting part of your character comes from
after you start play. If you're starting at first level, the majority of your story is ahead of you, not behind.
As to your first point, one of my rare times as a player has indeed informed how I DM. My character died in a campaign, alas, and I started to roll up a new one. However, I was on hold for a real month until it made narrative sense for my character to appear. The party had to leave the dungeon and trek back to town. Now, I could roll with needing to wait for a week until next session to get folded in, but indefinitely? No. After two weeks I found another game. Since that point, if you are present you're in. Just rolled up a character? The party finds you locked up, about to be eaten, fleeing, chilling at a camp, whatever. Similarly, if a player can't make it their character is in quantum superposition until next session. They can't do anything, really, but if they have a particular point of view, contact, magic item that is particularly important it is accessible.
I do tend to start new characters at first level, or half level of the average once the party gets to 6th level or so. It's at that point the AoEs come out and they need to survive one hit. Characters are eligible for xp with participation, and an attack roll my or against them counts. As I use an AD&D-ish xp table, levels come pretty quickly. Plus with treasure and achievement based xp as well they're up to speed soon, and have a history that's developed through play.
So, what happens if a character dies? Divine raising of the dead is possible, but often a character is maimed in some manner, or another form of loss occurs (ritual completes, prince lost, unconscious and the money train escapes). If a character loses an arm or eye, then they can gain an automail arm that can act as a shield or goblin's eye that also lets you see curses. Having some kind of magical, alchemical, or artifice solution is carries as soon as they can afford it so the party can take their mangled friend and escape.