@TheWheelie
from a conceptual level an underwater defense unit should not have that much hp in general.
From a conceptual level I would expect a defense unit to have more health than a non-defense unit (which is also consistent with static AA, T2 arti and PD vs their mobile counterparts). So presumably your point is an underwater unit should not have as much HP as a surface unit (something I'd agree with).
However, the Atlantis is a fairly big contradiction to that - to be consistent, given it's already being changed in this update, should it have a significant DPS boost and health nerf to be consistent? It currently has 40k health, almost as much as a battleship, and far more than it's closest comparable unit, the T3 Seraphim subhunter.
Meanwhile in terms of the severity of the nerf/your point it could always be buffed if it's too big a nerf, much higher priority changes have taken a while (e.g. GC), so I think it optimistic that if the HARMs was nerfed into obscurity it would be a priority for a buff in the near future. Better to nerf moderately and then if it's still a problem to nerf again than to risk making the unit near-unusable.
To illustrate why it looks too big a nerf - I would expect an immobile torpedo defence unit to be a mass efficient counter to a head on attack by torpedo units (indeed previously when I've called for a buff to T1-T2 torp launchers due to their weakness against surface naval units I've been told they're meant as a counter to subs, not surface ships). From a general balance perspective it is sacrificing mobility, and is a higher tech level (so harder to obtain), so should be compensated to some extent for this. Meanwhile the relationship seen on land where T1 units are mass efficient against higher tech units is less relevant as it's simple to concentrate a large number of submarines in a single area (achieving a similar 'force concentration' benefit to higher tech land units) - the same can't be done for land units, where 100 t1 tanks won't all be able to engage at once, and where aoe is much more prevalent.
So, doing a simple sandbox test of spawning T1 subs out of range of a harms and having them attack it, 9 T1 subs (the mass equivalent value is 8.333) bring a HARMS down to 3.7k health.
In other words, even before the range nerf is factored in, T1 subs will mass for mass trade roughly equally to the proposed harms. That seems a nerf too far, especially given HARMs is outranged by T3 surface naval units (battleships) which can kill it for free (and more easily than before with the combined removal of stealth, reduction to HARMs range, and reduction to health).