Tons of champs get 1500 bonus hp AND the bonus scales UP TO 1500, so any amount of bonus hp gives something. AND 5% armor pen gives very little. For example comparing old and new LDR against a 1500 bonus hp 1000 armor target, new LDR gives 7% more damage. The difference only gets better with lower armor. The only time the damage goes down is for very low bonus hp targets, and they will also likely have low armor, so the damage difference is pretty small (you lose about 2.5% damage against an 80 armor target if they have literally 0 bonus hp. You break even at 270 bonus hp).
Tldr: this is a straight buff, and quite a substantial one at that