D&D has always been anti-deathspirol at its core - you didnt get gradually worse with innevitable death as an individual hit points acted as a buffer preventing it ...
However death spirols still exist in D&D how is that possible? Easy its the party Death spirol where a character death weakens the over all party and generates a cascade failure.
4th edition has increased intra-party synergy some and this is very cool in so many ways and appropriate however it does increase the likelihood that one players character going down can take ot the entire party because the whole is indeed greater than the sum of its parts.
TPKs are generally not fun and for the most part they mess with everyones story DM and player alike... removing synergy and dependence isnt a good solution its like removing a layer of all the time fun trying to prevent an incident of un fun.
My solution involves a Trope called the Conservation of Ninjutsu = I wont link to tvtropes.org that is evil but I will employ plausibility deniability to let you do it to yourself.
Note In fiction this occurs amongst the heros not just the adversaries a group of - You see what happens when one hero goes down? they get flash backs of relationships and moments with that hero they become emotionallly inspired etc. They are enraged / or driven..
This effect doesnt necessarily happen all at once after all everybody gets there turn for a spot light... and it is often a reflection of the hero how he affects his allies.
Solution in Simplest Form : The player can when there character is down / disabled operate in a fashion like a defacto LazyLord creating various support effects and maintaining the action economy of the fight. It keeps the player involved in the tactics of the fight.
The idea is just because your character is out of the fight you the player are not.
More than One Bird with a Single Stone.
Players don't like characters being disabled for long durations make sense as it takes them out of the play - deprives them of choices removes there vehicle of influence.
However with all the deadly tools being employed sometimes it also makes sense within the narative for this to happen... by finding ways to allow the player to influence play even while ther hero/avatar is disabled. We solve multiple goals.
This concept improves the acceptability of longer term character disabling effects whatever there flavor...
One element of implementation might involve giving characters powers which require they be disabled or down to varying degrees.
Heck you might establish a special relationship with another hero so your two characters could use powers that are normally restricted to a downed hero could target one who is just disabled.
May your story never be ruined by a tpk.