How exactly do you kill - or at least disable - PCs?
As far as I can tell, when PCs drop to Flesh Wounds, they are still active. When they lose stats, there are no listed penalties for dropping below 0 in a stat - so theoretically, that adroitness -20 PC might still be flopping around ineffectually.
I've noticed the rule which states that if all the damage successes are a success, the PC is "Smashed" - a process which looks pretty difficult (even if you do reduce Wherewithal to zero - especially if armour is worn).

I'm considering a house rule which says that when all Flesh Wounds are gone, damage comes off Wherewithal unless stated otherwise, and when that is zero, unconsciousness occurs.

PC death isn't supposed to be a frequent part of Donjon, so the smash rule is the only way to kill a PC.

Keep in mind that smashing a PC can happen as a result of any attack (once FW falls below 0). So while you're whittling away at attributes, you have chances to smash the guy.

That said, there is a flaw in this rule. A very powerful monster (let's say with 20 dice) is basically going to get the whole range of numbers in his roll (from 1 to 20) every time he rolls, so he's almost never going to smash anyone (since a 0 attribute still lets you roll 1 die). On the other hand, a pathetic monster with a 1-die attack has a pretty good chance of smashing you on his high rolls.

I think your house rule is probably better, though I think the intent of Donjon is to have the GM use facts to do things other than kill PCs. For example, I'd start using facts to tie up (reducing key attributes) the PCs, and then have them hauled off as prisoners or the like.