We don't live in a 1e or 2e world, either. The overwhelming majority of people who played those editions did it in the prior era where that playstyle WAS common and desired. Furthermore, most of those who play 1e and 2e today are grognards who still want to play that way, which is why they are...
Most wizards never got to that level. Not because play stopped like @Lanefan was getting at, but because at level 1 they typically had 2.5 hit points and an AC of 10. They'd be knocked out if the PC in front of them stopped suddenly and they collided.
Even if they did make it to 17th level...
Given WotC's recent history of using the UA to advertise stuff already set in stone, it's entirely possible that the most recent UA stuff will be in it. Death Knights and Liches are right at home in that place.
It's a Death KNIGHT, not a Death Tracker or Death Sneaky Guy. It's geared towards fighters and paladins, mainly paladins as @Paul Farquhar pointed out. As for the spells, Death Knights have always had spell like abilities and the path feats are designed emulate that.
Maybe you just have things you still want to do and don't want to die. No need to think the world can't get by without you or that you're doing it for the greater good. ;)
As for it being selfish, pretty much everything we do is selfish. We eat because we don't want to feel hunger pain. We...
They're dead and cannot access any funds. The heirs who now have all the power could access those funds, but I doubt that very many would. They now have the power and funds, so why give it back? And that's before getting into souls that do not want to or cannot return due to being trapped.
Not really. Souls can choose not to return and many would just stay in the afterlife. Especially good people who are the most likely to have people who want to bring them back. The upper planes are idyllic for them. On the lower planes, demons, devils, etc. trap souls or turn them into...
If by completely new, you mean there from the beginning, you are correct.
The 5e MM says that liches feed mortal souls to their phylacteries.
The 5e MM says that souls that descend to the lower planes(abyss) are turned into Manes. Demon lords feed on manes, destroying them utterly. Soul...
I think you're asking for too much granularity. We already know that after dying the soul leaves the body and travels through the astral plane to the outerplanar destination. That's sufficient. The questions you ask should be up to the DM to work out if they ever come up.
At what point does...
I'd probably go with my favorite fighter, the battle master. For bard I'd go with the college of eloquence if that was available. If not, the college of lore.