One reason Canon's built-in magnifier disappeared is size.
The magnifier uses a rotating "turret" just in front of the eyepiece. The turret consists of a box with lenses on two sides - a negative lens on one side and a positive lens on the other. These lenses make up the optics of the viewfinder. When you switch the turret to one position, the positive lens is closer to your eye and the negative lens is closer to the subject; this gives a wider-angle view.
When you switch the turret to the 'magnify' position, the box swivels through 180 degrees, so that the negative lens is closer to your eye and the positive lens is closer to the subject. This makes the lenses act like a little telescope and causes the image to be magnified. (It's the same effect you can get through a regular RF viewfinder by looking through it backward.)
The third position swings the turret through 90 degrees, so you're looking through the sides that have no optics in them. This gives you a life-size 1:1 view.
All this swiveling means that the rangefinder's beamsplitter (the part that merges in the image from the secondary rangefinder window) has to be in front of the turret. That's not a big problem with the older Canons, which have no framelines (V-series and earlier) or simple reflected framelines (VI-series.)
But all modern RF cameras use projected framellines, which require a separate optical system to merge the image of the frameline mask into the viewing path. Having this plus the turret would add length to the RF system, forcing the camera body to be thicker. A lot of people already feel the 7-series Canon body is a bit chunky compared to the older models, so you can imagine how customers had reacted if it had had an extra 3/4 inch or so of viewfinder sticking out the back!
