A party can usually deal with 2 PCs being downed every turn as long as someone who is not downed has healing. If someone who is not downed has healing word one PC getting downed doesn't really even reduce the parties overall effectiveness.
Healing Word is really one of the most powerful spells in the game. It is a bonus action to cast so you don't even lose your action and you bring a downed guy back and he is not any worse for the wear.
I'm not following your math here.
Standard 4 person party. 2 PCs downed every turn. One healer (I'll be generous and assume the healer won't be targeted unless all other party members are downed first). I'll also assume side initiative, since that way downed party members won't have their turns skipped if they would act before the healer's turn. The healer will use HW on their turn to get an ally conscious. Since you can't also cast a non-cantrip spell on a turn that you cast a BA spell, we'll assume that's all the healing they can provide (most healers don't have class features that provide non-spell healing options). I'll also assume a powerful solo monster so that the damage remains consistent throughout the encounter.
Start: 4 acting party members (APM)
Monster acts: 2 downed party members (DPM)
Players act: 3 APM
Monster acts: 3 DPM
Players act: 2 APM
Monster acts: 4 DPM (probable TPK)
If the party wins initiative, they have 9 combined turns to take down the monster before the monster takes them out. If they lose initiative, they have a total of 5 turns to take the monster out.
Admittedly, if the players can kill the monster quickly enough, the fight might be winnable. But since HW only restores enough HP to be conscious (the next hit is likely to down the character again) the DPMs/rnd could easily increase if the monster has AoEs or multiple attacks, since it just needs to tap the HW recipients to down them again.
I'm not disputing that HW changes the dynamic of combats. It certainly does. However, unless you have multiple healers in the party, or a life cleric that can heal using a class feature, or the party has enough offense to kill the monster before it takes its 3rd turn, I don't see how you can down 2 characters a turn and not have it result in a TPK, even with HW. Unless you have some kind of house rule that allows a caster to cast 2 HWs a round, but that would be more an issue with that house rule than HW, IMO.