Coup de Grace not gone, it's just not explicitly called out.

Unconscious....Any attack that hits this creature is a critical hit if the attacker is within 5 feet of the creature (PHB 292)

This same wording is also used for Paralyzed.

You don't have this mechanic potentially leading to automatic death (though it can with the already in place death rules), but it is an automatic critical hit which should stack on a good bit of damage.

So how do you deal with a helpless opponent? If they are paralyzed or unconscious, you can hit them and get an automatic critical (And you have advantage to attacks against them too).

Coup de Grace is not gone, it's just not explicitly called out.

Unconscious....Any attack that hits this creature is a critical hit if the attacker is within 5 feet of the creature (PHB 292)

This same wording is also used for Paralyzed.

You don't have this mechanic potentially leading to automatic death (though it can with the already in place death rules), but it is an automatic critical hit which should stack on a good bit of damage.

So how do you deal with a helpless opponent? If they are paralyzed or unconscious, you can hit them and get an automatic critical (And you have advantage to attacks against them too).