Before suggesting nerfs to the amulet itself (I agree that might stacking is a bit too high from sigils and runes combined), we should first understand why was the celestial amulet so important at adding build diversity to the ele class.
While some other classes can rely on strong active defenses without any investment on defensive stats, and therefore focusing on amulets like zerker and be top tier, or have enough condition damage to go high dps and still invest on defensive stats, the elementalists active defenses are heavily reliant on stats, especially because of healing (healing power + toughness), because of auras (which require us to get hit, therefore vitality + toughness) and boons (boon duration), and our damage is hybrid and hard to specialize at.
Celestial allows balanced/ offensive eles to be viable with the meta, without relying on instant bursting tricks and gimmicks to compensate for their ineffective defense. I would say that celestial pretty much goes hand in hand with the profession’s design. And then, battle sigil and runes of strength allow celestial eles to focus towards a path – in this case, of damage.
The problem with nerfing this, is that the current style for eles actually feels like what anet intended for it to feel like all this time: where “every” skill is meaningful and impactful. Where your active defenses are more than delaying your death for three seconds while being targetted while in zerker gear. Where your hybrid damage is actually meant to be useful and not detriment to your potential. Where your burst skills are actually meant to burst, without coming at the sacrifice of your defensive half of buy gw2 gold.
This makes it complicated for Anet to “just nerf” this amulet, because the risk of screwing the entire profession is high. Celestial Amulet is the second half of the elementalist class, design-wise, and toning down sigil of battle and rune of strength would make it really hard for eles to spec offensively without sacrificing the only amulet in the game that makes the entire profession feel “complete”.