I mean some of them show their age a bit and ok some of them rehash ideas from preceding ones, but it's hard to think of any Miyazaki movie that did nothing for me at all.

One of the most recent ones I watched was Whisper of the Heart. It can be summed up as "girl meets a cute guy, girl wants to be an author, writes about a cat in a parallel universe, finds an antique shop, random proposal at the end", it was like watching a clipshow. I remember reading reviews for Totoro and them trying really hard to beat around the bush with "ah it doesn't have a cohesive plot, buuuut..." and then the rest of the review, almost like they didn't care because it was Studio Ghibli. I've seen movies panned for that (e.g. Alita Battle Angel or Spiderman 2).

I think Mononoke was easily the weakest of Miyazaki's movies but it's the one everyone raves about because it was the first one to see a widespread and non-butchered release in the West on DVD.

The earlier Totoro, Kiki, and to a lesser extent Laputa are all better movies, in my opinion. The former two shine exceptionally by being charming slice-of-life vignettes from a time before that sort of thing was the mega genre it is today, managing to be captivating stories that somehow don't need or contain any kind of villain, quest, or cliché call to adventure whatsoever.

Laputa does -- in spades -- but it's still great. It's got so many villains that it's got two sets of bad guys, but one set of bad guys turn out not to be bad guys and basically the entire damn movie is a flying scene of some type or another and it's fantastic. Castle of Cagliostro is also awesome, and it was arguably one of those pivotal Citzen Kane style moments for the medium that turned out to be surprisingly influential to subsequent works, both animated and not. (And also introduced an entire generation to the only version of Lupin who is not an huge asshole, much to the surprise of everyone who watched this movie first and went on to check out... any... other Lupin III works.) Cagliostro was so influential that Japan's (former) Princess Sayako based her real life wedding dress off of Clarise's dress from the movie, and said so.

Have you ever watched Alita: Battle Angel?

I mean in general, not really all of them (for example, Spirited Away gets honorable mention on every list). I remember reviewers for Totoro trying their hardest to not spotlight the fact it has such a jumbled plot when movies have been panned for that before. A masterpiece is supposed to impart something onto someone, but except for Spirited Away and arguably Marnie, my main reaction was little more than "well now I can say I saw it".

Being all their ages didn't change a lot for me (not sure how normal that is amongst those age groups). Except for Totoro which I watched when I was four if I remember correctly, I was roughly a preteen to teen when I watched all of them (or all the ones I watched, which is all but three) up to The Boy and the Heron (which just came out, I'm an adult now) which would only put me out of range with Ponyo and Porco Rosso when I watched those. Most people have a good sense of feeling for a story that adds up. I was little when I watched Totoro and little me had to stop myself from getting distracted from the movie itself.

I don't think that ruins it for me, rather the opposite. Ashitaka is a member of a tribe far away from the places in the movie. When he gets there he is just an observer to the war between industry and nature and wants to form his own opinion. He gets sucked into it and even if he did nothing the story would have continued almost the same, bar the ending where he then has made up his mind. I'd have to watch it again and spare more than a few minutes before sleepy time to write a better response, but those are my two cents

By this logic, Raiders of the Lost Arc, is pointless. Still a great movie, but Indy does nothing that changes the outcome.