Why is that?
What would keeping the creature the LG alignment offer to the game? What makes the creature's alignment being LG so important? Why is changing it from LG to unaligned so problematic for you?
110% Archetype.
D&D, for me, more than most things, is a game about fantasy archetypes. The strong warrior, the stout dwarf, the brilliant wizard, whatever. D&D tends to hodge-podge and throw in a few of its own, but rarely is a fantasy archetype ever expressly written out.
The "good dragon" is a fantasy archetype. The dragon of light and purity, aloof and defending the innocent. This is a fantasy archetype, contrasting with the evil dragon of smoke and fire, it is a dragon of sunlight and maybe even 13-year-old "I want a dragon buddy!" ideas. It's Falco from The Neverending Story. It's, heck, half of dragons in fantasy these days, because people want to be buddies with heroic dragons. D&D may have even had a hand in creating this archetype!
"Oh, but you can't really fight and kill it!" is a horrible reason to violate archetype. For me, this isn't a game about fighting and killing things. I don't care if I can't fight and kill very many LG gold dragons. They don't exist for me to fight and kill, generally speaking. They exist to help my group tell our story, to make our world more interesting and engaging, to add a variety of challenges and allies to the mix...a dozen good reasons for a dragon of pure goodness to be present in the rules.
The fact that they should exist and should be dragons of pure good is also more fuel for my "Mosnter Manuals should not just be stat blocks" point, too.
In short, I want gold dragons to be LG
because it makes my D&D games play better.
Yet another stat block to reduce to 0 hp doesn't really do that.
And, yes, I know it's "easy to change for your campaign," but that's not really the point, now is it?
Coutls irk me, too, but they're far enough removed from their source material that it's an annoyance only. This annoys me a bit more than the inability to make an effective "Intelligent/Dexterous" rogue (though it's on the same continuum), largely because that's a mechanical gap, while this is just a bad decision for the direction of Monster Manuals in 4e. The former might be patched, the latter is going to irk me probably every time an MM comes out, when they decide to make Joan of Arc, Mother Theresa, and the Baby Jesus unaligned just so you can fight them.
Insane human nobles irk me for entirely different reasons.
I'm not that attached to pointless legacy -- I'm not a fan for pointless change, but I don't begrudge experimental toe-testing either. What irks me is that there's a great fantasy archetype out there that millions of people are familiar with that they just turned into a house rule. Way to go, jerkbags.