Do you recall where you found the recipe for apple streusel cake? I pulled up tons of images from search engines and Pinterest and none of them looked like the image you posted. There was one that looked interesting but I got a 404 when I tried to pull it up. Was your cake as good as it looked?
http://www.musingsofagem.com/2013/10/apple-cake-with-streusel-topping.html
I made it a couple days ago. One Gala apple had been in the fridge too long, wasn't spoiled, but got a little mushy after peeling and sitting in cold water for awhile so they wouldn't brown.
The cake sank a little in the center, but maybe I didn't bake it quite long enough. It's hard to test with the streusel on top.
Now I was disappointed with the streusel; it wasn't clustered and slightly crunchy or slightly browned looking like hers. More butter might make it too heavy. But if I made it again I would add a couple more tablespoons of butter and cut it in cold with a pastry blender, leaving it a little chunky. Or something. Mine tastes ok but is too grainy.
I used buttermilk for the cake part. That worked out well.
If you decide to try it, maybe you can come up with something a little better. Use firm but not sour apples.
My daughter likes it. It's certainly fit to eat, just really disappointed in the streusel top.
Also I thought maybe I would make it in my big tube pan if the center keeps sinking. It almost fills a 9-inch springform pan. Otherwise I like the look of it just the way it is.