Smartphones/iPads (not tablets in general) have gotten really so good and at the same time so expensive that is hard to justify the frequent upgrade. It has gone to the point where smartphones are so great that is hard for companies to make the new models appealing enough to make people upgrade. And it is not that I find the new models boring. It is because I find the one from last year I am using so good that I don't feel I need to upgrade for the first time.
I don't think this is bad thing at all. You don't need to keep people on the treadmill - just in the gym. People will get on and off as they wish, but so long as you're selling them apps, music subscriptions, iCloud storage, etc you're making more money on what you've all ready invested in. Think of it like this: a buck for 50gb of storage, 10$ for Apple Music, a few 2$ apps a month, you're pulling in 15$ a month without a single dollar more investment. While Apple will always be a hardware company there's plenty of money to be made in that big services division and that will be best served by keeping people with Apple devices in their hand.
Now, Apple needs to keep people in the gym by keeping up the speeds on reasonably aged older phones, like they have in iOS 12 - I'm shocked how nice it runs on older hardware like the 6 and 6s. I'm very pleased on my 7+. I don't feel an incredible push to upgrade (beyond the fact I sell Apple products for a living and love having new things... but realistically it's not going to revolutionize my life like getting the 7+ from the 6 did).
Apple has lots of revenue streams and as an investor I'm insanely happy. Just keep things rolling. I'm happy with the change to OLED screens and I'm happy they've given an LCD option with the vast majority of feature parity. I'd be happier if AT&T weren't so bleeping expensive and might cut me a deal for selling their services, but Que Sera, Sera.