Magic missles is instant death. Or you use your NPC's bonus/secondary attacks or the minions do it.
Very few combats involve magic missile, so the odds of there being a foe with magic missile available, who knows to use it the instant a foe goes down, is vanishingly small. There are a lot of assumptions being made about the number of foes, attacks, their priorities, and the initiative order. I note that many, many people have commented on this problem in 5e, so, sure, you can be contrarian and argue that it's not really a problem and most folks are just bad players, or you can consider that maybe all those folks have a point. At least about their games.
Always debatable. And it's not the healers they have to go after, but the one that was down and healed back up. It's pretty easy to know they are going to be easy to knock down again. And once you do that you are going to want to keep them down.
Sure, if it happens once most foes will start to get wise, though a lot of fights involve unintelligent creatures, beasts, etc. so it really depends on the context. But for the most part, combat only last a few rounds in D&D, so all you need is to get someone up once.
I try to RP each combat from the perspective of the mobs involved, keeping in mind that death saves are not normal (most things are out of the fight when they go down), and the mobs making the decision are typically still fighting for their own lives. There is a lot happening in each 6 second round. If you are fighting a rogue and a barbarian and the rogue goes down - maybe dead, maybe unconscious - but the barbarian is still swinging their great axe at you, that's going to focus your attention.
They all take an action or bonus action. But agree (if I understand your point) that healing still takes an bonus/action whether you do it before or after the PC goes down.
Action and bonus action are vastly different in terms of action economy. Bonus actions get wasted all the time. They don't let you make a regular attack, and so on. And healing word has a 60' range! That is so much more powerful than cure wounds that it doesn't compare - I have no idea why anyone would bother with cure wounds - it does fractionally more healing but uses your actions and has no range. Not to mention many, many other abilities that can pop characters back up or even mitigate damage as a reaction.
That's why
most of the time healing before someone goes to 0 HP is super inefficient. And death saves are an extremely valuable resource - each of them potentially represents a huge amount of avoided damage. Thus, experienced players gauge the situation and use them.
Obviously the game is not black and white and there is a time and a place for a clutch heal. And a few high level healing spells are often a good first choice (heal, mass heal, maybe mass cure wounds). But mostly, healing before someone drops to zero is a very bad call in terms of resource management.
Depending on the table. For example, if you have a very adversarial DM who likes to play mobs like they have all of the DM's information, so as soon as someone goes to 0 every mob drops what they are doing and swarms the fallen character, then yeah, change your strategy. I consider that meta-gaming and don't do that, but I recognize that some folks enjoy a much different style of play than I do. But in the game as it actually happens at most tables, yo-yo healing is widely considered a problem.
Edit: so if it's not an issue at your table, then bless. But there are folks in this thread trying to argue that, in effect, it's only a problem because most people are playing the game wrong, and that's a bit condescending.
