I think the reason photography hasn't really changed at all in the last 100-150 years is simply we like it as it is. Sure people go off and do something a little differently, but really not much changes. The tools changed, but the subject matter is pretty much the same. I'd argue the reason is, is because it fits us, it suits us, it does what we want it to, and we like it.
Take a tool like a car. Invented 1800s, but it does exactly what it did when we first invented it. It moved us from one point to another. It got fancy but didn't really change.
A camera is much the same thing. It was invented to capture and record our lives, and events in our lives. No one has really been successful using it as a maker of art like a painter/canvas does. We obviously have arty photographs but that's mainly because it follows the same rules/guidelines of composition that we've come to accept as tickling our fancy, from the painting world.
I think the reason for that is, photography is pretty happily in the nitch it was created for and nobody has found a new use for it.
I don't see that as stagnation. A piano has 88 keys. Been that way for a lot of years. Lots of different ways to play it, untold combinations and styles, but in the end, it still sounds like a piano, no matter how you play it. Has it stagnated? Has it become boring? Does placing tacks on the hammers and making it sound like something else advance it in any way. Not to me.
I find photography the same way. Incredible tool. The results can still move people to joy, tears, and every emotion in between.
I think it's kind of perfect.