Why even bring that up though? She is quite clearly successful now. Why even try and tarnish that by saying she has now set high expectations and probably can't meet them? First of all, isn't setting a high bar a good thing? Secondly, who says she won't meet them? She may or she may not, but a Grammy win is a pretty good indicator that she is the real deal. It is on that point that your comparison with Ben lummis falls over, you're comparing two completely different universes of achievement. Absolutely nothing suggests that because Ben lummis faded from the limelight that lorde will too.
At the end of the day, She doesn't really need to prove anything else to be honest. She already has a Grammy win, which is something many artists aspire to, and she has achieved at the age of 17. Good for her.
Her music isn't my sort of thing either, but you have to appreciate the achievement surely.