From all what I've read about in 5e so far, it is indeed pretty easy to bring people back from the dead. Although even True Resurrection has limits: someone who died of old age, someone whose soul is not available, and the person has to be willing (which I guess PC generally are...).
However, the type of play you get at a higher level (starting around level 5, actually) is not so much whether the party will survive all battles but rather whether they accomplish their task. Say they have to kill those lycanthropes (or better: remove the curse on each one of them, which means keeping them alive and casting a spell on them to remove the lycanthropy...) before all in a village get infected. Success is not just being alive and getting rid of the lycanthropy. It is also that no more villagers get caught by the time you're done. It is getting things back to normal for that village. Maybe this village has no money or any kind of good to offer to the players but the players still have to help... because if they don't something much worse could happen.
When at level 10+, you are expected to deal with politics at a greater scale too. So your younger self protected a village, now you have to protect the whole Sword Coast by gathering information, finding specific items and avoiding the collapse of the order of giants of the North... Not succeeding would mean a very destructive war with the giants. So your players may never die, but they may end up seeing death in front of their eyes that should not have happened had they succeeded in their task!
When attacked by ghosts, you may lose 1d4 x 10 years of your life. This will kill you of old age and I have not seen something that reversed such damages (there is the Potion of Longevity that can help a bit, but after you drank one, chances are you will age instead!) If you're an Elf, you're probably good. If you're an Aarakocra (Element Evil Players Handbook—their age limit is around 30 years!) then you can die by one Ghost touch...
When transformed to an undead (shadow, revenant...), you have a problem because then your soul is imprisoned within the undead creature. Obviously, destroying the undead will be enough to free the soul, assuming you know which undead we are talking about (if transformed to a lich or mummy lord it is a bit more complicated, you have to destroy the actual heart, although such transformations generally do not happen without a lot of prior preparation so it is not likely to happen unless a bad guy captured your player character...)
The Soul could also be imprisoned in a magical item or on a plane.
And in most of these cases, they cannot be found with a simple divination. So a bad guy captures one of your players, kills him, transforms him in a zombie and releases the zombie in a grave filled with such...
A god can be destroyed when killed on his plane of existence. To my point of view that means its essence (soul) gets destroyed. This would mean that the soul of a player character could also be destroyed. Yet a rather complicated task... Also there is nothing about the possibility of destroying a soul in any D&D that I've heard of.
There is one other, rather terrible solution: all party members die. When that happens, it is really unlikely that anyone will bother bringing these heroes back.
From what someone else mentioned, having enough burden in bringing someone back from the dead (i.e. it takes 1 hour to cast Resurrection and the cleric may not have any more spell slots available to cast the spell today) could mean that the resurrection cannot be cast right away and by the time it could, maybe the cleric is dead too... the rest of the party runs away and gets in front of other monsters and gets killed one by one that way. If one person makes it back, he may have lost all his gold and diamonds by then... What if he's not the cleric? After some years, when he has the money again, will he still be thinking of his friends and pay to bring them back to life?
I've also heard of really unlucky parties all jumping in a portal of annihilation at the same time or entering a room that will freeze them all to death.
In other words, they should still keep on their guards, but a direct death, when well prepared, can be absolutely acceptable since it can relatively easily be reverted.
Really, in most of the cases I've seen where people died it has been a total destruction of the party. In all other cases, they get healed at some point and go back to adventuring...