Yes, it's D&D. Your PC might die if they're unlucky from time to time. And there are cures for that, too. Honestly, this game is not 'rock-paper-scissors' where every move has an easily-available equal and opposite counter. If you have a PC who dies because they didn't take Dispel Magic at 5th level and were hit with a powerful, persistent Heat Metal then perhaps you take it from then on, or at least a scroll?
You say that the game is not rock-paper-scissors but then you go on to explain that the answer to someone throwing Rock (Heat Metal) is to throw Paper (Dispel Magic). My point is precisely that it SHOULDN'T be Rock, Paper, Scissors. If they throw Rock (Heat Metal), I should be able to throw anything I want back and still be effective. If I don't have Paper, I should be able to throw Rock back or Scissors or Wood or Escalator for that matter and still have a chance to win.
Part of the change they made to the game was that every spell that causes problems for a long time has a save every turn to go away or to only have a fairly minor effect. That way no matter what spells the enemy casts on you, you don't need to have a specific counter for it. Someone can cast Hold Person on the fighter and it's likely to last one round before they can move again. They can cast Bane on the fighter, but he'll still be able to hit with only 1d4 removed from his attacks. Heat Metal is a major effect (2d8 damage per round while essentially giving -4 to -5 to all attack rolls and ability checks) with no saves after the first one.
It should be also noted that Heat Metal is a 2nd level spell, as I keep noting. Dispel Magic is a 3rd level spell, so you can't even use scrolls of it until you are 5th level. Monsters won't have Dispel Magic prepared 99% of the time. It is too powerful in either the PCs hands or the enemies hands.
And there isn't a spell that kills everyone at low level. That's facetious. Heat metal only endangers those in metal armor, who typically have a fair amount of hit points. If someone is roasting due to the spell then yes, you hit the caster with everything you've got. Or maybe the PC jumps in a pool of water in hopes that the DM will reduce the damage while it's in effect. Or the DM allows a wizard to cast Ray of Frost on the armor each turn and allows the damage from Heat Metal to be reduced by the amount done by that spell. Or the PC dies and you try again.
It doesn't affect everyone, that's true. But there is almost always a valid target in the party. And that target gets hit with a spell that's more powerful than all the other 2nd level spells. That's my point. If you know there will be someone with armor in the group, why would you prepare any spell that isn't Heat Metal at 2nd level, since they are all worse than it?
And monsters can just hide. The first time we ran into this problem, we were all 2nd level. No one in the group could cast Dispel Magic, we weren't high enough level. The enemy Lizardfolk Shaman cast a Heat Metal on our Fighter with 20 hitpoints, then immediately hid behind a wall 80 ft away from the party. We were in a swamp where it was difficult terrain so we could only move at half speed. We couldn't target him with any spells or ranged attacks because he had full cover. Also, there were a bunch of lizardfolk that ran up and engaged us in melee the same round, so leaving melee to go after the Shaman would mean a bunch of opportunity attacks.
At an average of 9 damage per round, that means the Fighter had 2 rounds to stop the spell or he was unconscious(since the damage is dealt on the enemies turn and the first damage happens when the spell is cast). If it rolled 1 point above average twice then the fighter only had one round. And he had to try to hit the shaman with disadvantage on his attacks. What's worse is that while the fighter was unconscious, there was no way for them to stabilize since they were taking continual damage and auto failing death saves.
It was pretty much guaranteed to take down the fighter. If the Shaman had cast any other spell in the PHB that was second level, the fighter was likely fine. A Scorching Ray needs to roll to hit 3 times, which had a good chance of failing since they were against the fighter. If only two attacks hit for average damage (14 total), then the fighter doesn't even drop. Melf's Acid Arrow averages 15 damage. None of those spells have the extra effect of giving disadvantage for as long as the spell continues.
The power level of a 2nd level spell should be doing about 14 points of damage with no secondary effects. This spell will last 95% of the time at least 2 rounds. That means it averages 18 points of damage WITH a secondary effect. Without a save every round, this spell is just way too good. If it's cast on a weapon that can be dropped then it's perfectly fine (and perhaps a bit weak) as a spell. But on armor, it lasts too long and there are too few ways to stop it.
I solved this problem as a DM by refusing to target armor with the spell so it doesn't feel unfair. But that doesn't mean it isn't a problem.