Drain magic

[Bolt Thrower]

I'm with Dalamar here. Spells are fairly clearly measured in time such as "for the rest of the game". That measure of time indicates that the spell effect has a window of time that it is active. Since a time of effect is defined, it stands to reason that effect is in play and can be drained.

[Iluvatar]

There is this difference (in my eyes):

Lowered Toughness/Armour Save stats come into play in every relevant dice roll in following turns (or phases). It has an active lasting effect on the game, Drain Magic cancels all effects of magic spells in play (on its target unit). So it all comes to this: are the altered rolls effect of the previously cast spell or not? (Edited to add the question as I am interested in possible answers and reasoning behind them.)

Lost wounds I believe don't have any active effect on the game. You can regain them or lose more as per usual, but the magic effect is no longer in play. You do not lose a wound a turn as the result of the spell => there is no ongoing effect of that spell => Drain Magic has nothing to cancel here.

Lost wounds have some sort of active effect on the game: it makes some abilities usable (any healing - Life Magic, Apotheosis, undead healing, Soul Feeder...). It also makes it easier to remove a model later.
But cast 3 times Wither on any T3 unit, and you can remove the unit - they're all dead, same as if they had all lost their profile W. There's no difference in the handling of those 2 stats in the rulebook (T and W): if they come to 0, the model's dead. The T also serves for other purposes, but it's not actually relevant here.

That said, I believe Dalamar has the exact criteria that should be used for Drain Magic: is there a duration specified in the spell? The confusion comes from "until the end of the game", "permanently" and no duration specified - all mean the same thing in most contexts... except for Drain Magic, it seems.

[Aicanor]

Lost wounds have some sort of active effect on the game: it makes some abilities usable (any healing - Life Magic, Apotheosis, undead healing, Soul Feeder...). It also makes it easier to remove a model later.

These are by definition passive effects, not active. But it doesn't matter that much I guess. There is still room for more than one opinion.
I quite agree Dalamar has it right, explaining rules from rule definitions (for example can Rust leather armour ir even the fact a model is sitting on a mount, so it is not a good idea to go into 'reality' arguments here, unless we want to get into really absurd ends - could be fun though ). I also think that Drain Magic RAI are there to enable removal even of those spell effects that can't be removed by dispelling. So I would go with that as the effects of the two or three spells in question can be removed without causing any problem in the flow of the game. If the T is reduced to 0, model is still removed and DrM can't do a thing as the effect is resolved before the spell can be cast. And if we ever have a FAQ on this, good - it may go both ways.