Though it can look pretty neat of you polish it up and acid stain it.

For decades, the marketing message was that less defects = better = more expensive. Apparently, when lab-grown diamonds came along, that had to be inverted: now, lab-grown = less defects = "less character"

The whole thing is such an obvious marketing exercise with very little to back it up (as evidenced by the extremely low resale value of diamonds)

Exactly, at the same price yes the natural stone will have more 'complexity' and be lower-graded than the lab one as a result.

They don't.

If they did, it would be possible to detect the difference with perfect accuracy. Instead, the detectors made by those interested in pushing the concept of "real, natural" diamonds have a false positive rate of 5% looking for the inclusion of things labs could easily add if they cared to

If that's what you are after there are more interesting stones than diamonds for that, ie moissanite.

You can make some plausible arguments against glass. It scratches more easily and doesn't shimmer as much. But synthetic sapphire is the same league and costs a lot less.

The modern-day aesthetic of diamonds is just that they are expensive. They're not distinguished by utility, quality, or appearance from cheaper products. The ultimate status symbol, but also obviously a bit of an issue...

Diamonds sparkle a lot more brilliantly due to their high refractive index.

(Moissanite is even better, so it should be preferred over diamonds unless I’m overlooking some other difference in their attributes?)

But plain glass gems look comparatively bland when used as jewelry.

Because they don't look the same. Not even a little bit.

Glass doesn't sparkle.

Because the refractive index of diamond is higher than that of glass, which makes it look prettier and "sparkle".

Which can be replicated cheaply in a lab, and will be the moment they become desirable.