Deepening the front fork dropout I have an old 27" road bike in a long, lazy and cheapish process of restoration. I'd like to put 28" wheels on, which requires 4mm longer brake reach. The bicycle used to have fenders and the existing 27" rim is already a bit far from the fork crown (and from the caliper bolt).

I had an idea to try to deepen the fork dropout in order to get the wheel closer to the crown, there seems to be enough space to do that, and still keep enough to get the axle bolt tightened.

Is there any reason I shouldn't do that?
Yes- structural integrity and your own well being. 4mm is a lot of metal to remove in the scheme of things. If I were you I would be looking for a long reach brake caliper instead of modifying the dropout by removing material.