£5k in five years was calculated on a cost of £2.50 a cup (plus if you save you get interest. Piddling amounts at the moment but something).
I find the idea that a couple who've saved £5k + £5k in five years (e.g. aged 21-26) should regard £10k as insignificant small change extraordinary. It's just very different from the world I live in (which is a pretty comfortable world).
But I think that's the issue here, for OP's DD. For lots of well-off people, buying coffee every day is an affordable little treat, their normal. Probably one of many treats they can afford. But until you have become comfortably off, it (like all the other treats) has to be regarded selectively.
The real problem comes when, like OP's dd, you grow up with the idea that it is everyone's norm, so to be taken for granted, expected (along with the nice clothes, hair, make-up and lifestyle that the tik-tokkers and instragrammers show off, because they're either from well-off families, or faking it with the hope of making it, or have already made it far enough that they're being given a lot of free stuff to promote). That aspect, the fake reality lives of social media performers and influencers, hasn't been explored here but is pretty relevant to the teenager in the scenario described.