What you’re talking about really isn’t that iPhone is or was a fad. What’s really happened is innovation has essentially halted and it has become a commodity like all smartphones.
Everyone and their dog keep trying to force me to use their apps, like there's a mantra inside their marketing divisions that apps are the shit. Doesn't look like apps are over. The market is just saturated of "app for X". The hype of getting rich from zero through one app is over, but apps aren't.
VR movies was a fad. You can date a TV by it's "VR mode" feature on the remote to a few years. No one is trying to sell me VR TVs anymore. That's what a fad looks like.
Then the market blew up and a few big winners ate up all the profit and everyone else died on the long tail.
Today, iPhone exists, but people don’t even think about them much. They just use it.
Pestering someone now about your iPhone app idea is like pestering someone about your website idea back in 2015.