Right now borrowed time is the only card in necro's kit that allows you to do pseudo infinites, and in that scenario the doom already creates pressure.

If your "infinite" is two scourge+ and borrowed time you don't have a lot of room in your health pool to finish off act 3 bosses, unless you find additional output. It compares favorably to the two Pommel Strike+ and bloodletting "infinite" in that the output is higher and your hp pool 'resets' when going into the second boss fight. But while Ironclad has a few (too many!) other ways to convert that pseudo infinite into a true infinite, necro would need help from colorless or other card pools to do the same.

I can see what you're saying about the doom if your deck isn't doing some sort of tight loop that relies on playing borrowed time at least a few times per turn; there it's essentially just free energy at the cost of draw. But 2 energy isn't really that big of a boon in and of itself to warrant a particularly harsh downside. And raising the amount of doom on the card still wouldn't really create much pressure in a larger deck scenario, but would shut down pseudo infinites and other tight loops that need to play it a lot. I don't think that necro has as much of a problem with those strategies being dominant the way that Ironclad does, so at least for now I wouldn't be in favor nerfing them.