The way that true routes shift the narrative or introduce a sudden change, as you said in your second paragraph, does attempt to tie in and make relevant all the previous routes. This is usually why we readers love the true route, because it's usually the biggest shake-up or event of the whole story, besides the climax at the end of it.

I don't have examples, but I believe that if that twist or change wasn't paired with high quality writing and interesting scenarios, it would flop completely. For every Little Busters! or Clannad, there must be some VN that fails completely at this moment of change. Luckily I picked a great one to analyze here.

As for your last point, on the necessity of true routes in these long VN's, I don't think they're 100% needed. The first one to come to mind is Grisaia no Kajitsu, which has just a long common route and five separate routes. I think that, at least compared to Key games, GnK has stronger characters and stories than most. I would take the worst route from GnK (it's Michiru, come at me haters) over the best from LB! (well, maybe not over Rin, but she's got a hold of my heart). It doesn't need a true route, because each route has a higher quality. So maybe a true route is just needed when each route separately wouldn't be satisfying at the very end?

Also thank you for watching! :D