> I have tens of thousands of dollars of camera equipment -- mostly more than a decade old -- and there just haven't been advancements warranting an upgrade. A modern camera will be maybe 30% better than a 2012-era one in terms of image quality, and otherwise, will have slightly more megapixels, somewhat better autofocus, and obviously be much smaller by the loss of a mirror. Video improved too.
I thought the same thing, and then I went and rented a Nikon Z8 to try out over a weekend and I was blown away by the "somewhat better autofocus". As someone who used to travel with a Pelican case full of camera gear, to just carrying an iPhone, I'm back to packing camera gear because I'm able to do things like capture tack-sharp birds in flight like I'm taking snapshots from the hip thanks to the massive increase in compute power and autofocus algorithms. "Subject Eye Detection AF" is a game-changer, and while phones do it, they don't have enough optical performance in their tiny sensors/lenses to do it at the necessary precision and speed to resolve things on fast-moving subjects.
In terms of IQ, weight, and all that, it's definitely not a huge difference. I would say it's better, but not so much that I particularly cared coming from a 12-year old DSLR. But the new AF absolutely shocked me with how good it is. It completely changed my outlook.
I say this, not to take away from your overall point, however, which is that a phone is good enough for almost everyone about 90% of the time. It's good enough that even though I upgraded my gear, I only bought one body when I traded in two, because my phone can handle short-focal length / landscape just fine, I don't need my Z8 for that. But a phone doesn't get anywhere close to what I can do with a 300mm or longer focal length lens on the Z8 with fast moving subjects.