I'm veering towards not so much extensions as a damn good blowdry, to be honest. And strongly considering buying some velcro rollers 
I also think part of Kate's polish is her makeup - she's a hell of a lot better at applying it now and she wears quite a bit. If you scroll down to the pic where they talk about her earrings here (whatkatewore.com/2019/01/15/kate-in-dubarry-and-chloe-for-garden-visit-the-designing-duchess/#more-56966) you also get quite a good look at her makeup.
Partly I think that's an age and experience thing (remember that eyeliner?) - I'm definitely a lot better now than I was at 20, I know what I like and what suits me. Partly it's having access to whatever products and as many lessons or even makeup artists as she wants - no cutprice mascara for Kate. (Although I doubt she has someone do her face for every engagement - the cost would surely be huge and it would be bound to leak).
Also, she wears more makeup than the women around her (I mean, how many people would do a full face before going to a gardening project? I wouldn't!) and does so regularly, so by comparison she will always look more polished than women who have either gone for their normal makeup or who have perhaps tried to up the game but have been there for ages waiting or aren't used to doing their makeup like that as a standard thing.
Obviously she knows she'll be in all the photos and has every reason to want to be well-presented so this isn't a criticism - just an observation / opinion.