On Upgrading to Full-Frame Cameras
Now that I’ve been shooting with full-frame camera gear for the past year or so, I'd say that the biggest differences I've found are that:
I'm much less likely to "miss" a shot for purely technical reasons (e.g. missed focus, blurry shot, blown highlights/blocked shadows, too noisy)
The images are marginally sharper, larger, and usually require very little processing to get to where I want them
The camera gear does a much better job at just getting out of the way and letting me focus on the image I'm trying to create
Could I use my old digital SLR from 2003 and still get great shots? Or even a small point-and-shoot? Sure, of course, but I'd be more likely to miss the shot in demanding situations (low-light handheld, fast moving animals, high dynamic range scenes) and I'd have to think a lot more about the gear.
My ideal camera, basically, is one that allows me to create what I'm envisioning and otherwise gets out of the way. The newer, full-frame mirrorless gear allows me to carry less weight, almost always shoot without a tripod, capture moving wildlife with really good autofocus capabilities and fast frame rates, and spend less time processing images since the raw files are so good. Essentially, the more the camera can get out of my way, the more room I have to think about the light, the composition, and the subject.
Give an old, entry-level camera to a pro and the most expensive one you can find to a total beginner and the pro will create better images every time. But, give the pro both cameras and the more expensive one will help them to make even better images in a lot of demanding situations.
Same thing could be said for instruments and musicians, computers and programmers, or knives and chefs. They're just tools to help you create what you want to create. Really doesn't make that much of a difference unless you know what you're doing and even then, it's marginal improvements that mostly just make your job easier by letting you focus less on the tools and more on the important things.