So you are advocating that a photographer not use the focus points? Seems a waste of a good tool to me. The focus points allow you to choose where the focus originates from as well as what tone you want to set the metering on. It's not complicated at all, just good use of all of the tools given to us in the camera. In digital photography when shooting RAW it's especially important to expose for the highlights. When shooting in JPEG one would need to expose on the brights, but keeping the exposure point more at the end of the midtone levels as there is not as much leeway in JPG. Why not use all the tools we have to get the photo we want without having to rely strongly on post processing: makes sense to me.

So you are advocating that a photographer not use the focus points? Seems a waste of a good tool to me. The focus points allow you to choose where the focus originates from as well as what tone you want to set the metering on. It's not complicated at all, just good use of all of the tools given to us in the camera. In digital photography when shooting RAW it's especially important to expose for the highlights. When shooting in JPEG one would need to expose on the brights, but keeping the exposure point more at the end of the midtone levels as there is not as much leeway in JPG. Why not use all the tools we have to get the photo we want without having to rely strongly on post processing: makes sense to me.

So you are advocating that a photographer not use the focus points? Seems a waste of a good tool to me. The focus points allow you to choose where the focus originates from as well as what tone you want to set the metering on. It's not complicated at all, just good use of all of the tools given to us in the camera. In digital photography when shooting RAW it's especially important to expose for the highlights. When shooting in JPEG one would need to expose on the brights, but keeping the exposure point more at the end of the midtone levels as there is not as much leeway in JPG. Why not use all the tools we have to get the photo we want without having to rely strongly on post processing: makes sense to me.

One of the main reasons to use BBF is to be able to set the autofocus and the exposure separately. So, lets say I am shooting a waterfall with dark mountains. I cannot focus accurately on moving water so I move my focus indicator just to the side of the running water, which will then be on the dark mountain, and I press my BBF to set focus. What I need to do now, however, is to set my exposure on the brightest part of the moving water in order to get a correct exposure for this shot. BBF sets and leaves the focus set unless I hit it again, which I do not do in this case. What I do is move my autofocus indicator button (in shutter mode this button connects auto focus and exposure), which is set separately from auto focus in BBF, to the brightest part of the moving water and the exposure will be automatically corrected. At this point I might take a test shot to see if the exposure works and perhaps take the exposure down a stop or two if it is too bright. So, you don't really need to have a separate exposure button with BBF, at least not in this type of situation.

Lots of answers, some humorous, but this is the complete answer, with page references to the manual. There are several ways to do this, and all involve reassigning functions to other buttons.

You can assign exposure lock to the shutter button. Custom Settings C1, Shutter Release Button AE-L - set it to ON to lock focus when you half-press the shutter. This is independent of how you have implemented BBF. (Page 336)

You can also use custom settings F2 to reassign the Fn button with a little more flexibility - you can press and hold to lock focus, press and release to lock focus, you can press to lock focus and exposure, etc. (Page 356) And you can assign the Preview button to do the same thing as the Fn button. (Page 372)

If you are having trouble reaching the AE-AF button on the back with your thumb, consider reassigning AFon to either the Fn button or the Preview button, then using the other front button to do AE lock. These two buttons might be more accessible to a smaller hand than the back button.

And no, you don't need to trade in your D750. At least not until it's replacement becomes available.