>The first is the complete lack of innovation. Without any meaningful APIs, end users cannot do much with their DSLR. There's no app store where I can buy apps to enhance my DSLR.
I disagree here entirely. The DSLR "app" market predates and has been the inspiration for all of these apps we see on phones. Photoshop filters, lightroom presets and actions, etc. All of the stuff that that inspired things like instagram filters have been around for decades now, and have consistently always been a step ahead on power and quality. You don't run them on camera, but... you wouldn't want to! When you're shooting with a DSLR you're shooting for consumption at higher resolutions than you can see on your LCD display. You want to be able to look at things in more detail than you could ever do on the camera itself.
>The other reason is much more banal. For various reasons, people jumped to the DSRL bandwagon thinking that would make for better photos compared to more compact cameras (false for most people)
You're right here, but I don't think people actually wised up to this.
If we're talking specifically about DSLRs, mirrorless cameras are just better for most people at this point - Sony has almost single handedly rendered DSLRs obsolete for the vast majority of use cases where people need prosumer or professional cameras. If we're talking about prosumer+ cameras in general, it's even simpler than your reasons: Most of the people taking pictures with their phones are doing so for people who are consuming them on their phones.
You simply don't need a DSLR or mirrorless or medium format camera when everyone viewing your pictures is also using a phone. People aren't getting photos printed, and they're not viewing them on monitors. I'm really impressed with my iPhone 11 Pro camera - it works fantastic! - for viewing photos on the phone. But throw it up on my 4K monitor, and the pictures don't compare at all to my old D5300, much less my a7r 3 or even my rx100 m7 (assuming good shooting conditions and manual mode - the 11 Pro auto settings are quite good and significantly better than the 5300, and in some situations seem to be about as good as the rx100). I haven't gotten any printed to put up on display, but just seeing how poorly they compare when I'm looking at them in Lightroom, I wouldn't want to.
The way the majority of people consume photography has changed, and the requirements for producing photos for that medium has changed with it.