The real question isn't whether guests will use them - it's whether your cousin cares more about impressing people or about staying within budget. $800 on candles is genuinely a lot, and I'd honestly suggest she consider a middle ground. A lot of weddings do favors that feel more like throwaway items, and guests absolutely toss them. But I've also seen people keep things that genuinely felt thoughtful or useful, not just expensive.
My friend got married a few years back and did these tiny potted succulents with little tags. Some people probably ditched them, sure, but I know at least three people who still have theirs growing on windowsills. The difference was the cost was maybe a couple hundred total, not eight. The gesture mattered way more than the price tag. If your cousin is set on candles, maybe she could do half the guest count with nicer ones, or switch to something cheaper but still personalized - like custom cookies or seeds or even just a nice jar with a handwritten label. People tend to keep things that don't feel like clutter.
Honestly, the tradition itself isn't outdated, but the "spend heavily to prove you're grateful" part of it kind of is. Guests appreciate that someone thought of them, but they also appreciate when a couple doesn't stress their budget over something that'll sit in a cabinet. If your cousin is worried about this, maybe just ask her straight up if $800 feels comfortable or if she's doing it because she feels obligated. That conversation might free her up to make a choice she actually feels good about.