AC -> HP balancing is tough because modelling impact is much harder than it sounds. Intuitively HP / (Hit Chance x Expected Damage) = Expected Hits to Kill so as long as HP / Hit Chance stays about the same it looks like not much has changed. Higher hit chance, lower variance, less frustrating, seems like an improvement. The problem is that damage is done in a small number of discrete chunks rather than deterministic blobs. One shotting weak enemies unreliably is much stronger than two shotting with ~double reliability because you will some times deny actions/placement (just existing in combat is useful, for example by threatening ranged attackers). And not all attacks are equal, opening attacks tend to be stronger, so making enemies more spongey nerfs that.
Also the flip side of lowering impact of variance on attack die is that it makes damage dice more swingy. For example part of why Warlock is so good in EA, 1d10 + Cha + 1d6 (hex and assuming Agonising Blast) is pretty good for one shotting at range, but even there a bad damage roll and your target surviving is more likely with higher HP, where as with the RAW 7hp a goblin has a 1/20 chance of surviving a hit with Eldritch Blast hit with +3 Cha, 1/60 at +4. With their current HP they have better odds of surviving a critical hit.
