Concavity is less than appears I think, the shadow angle exaggerates it (maybe not, they're pretty worn, 10 years and a hilly city). LBS didn't have a ball-end micrometer or anything else to measure. But I still knew it was getting low thickness, I didn't want a nasty surprise. Didn't want to replace wheels as I was hoping to get a disc bike (for this exact reason), so I moved the pads down toward the spokes where there is more metal and the rim is thicker and double-wall there; Pads hang off the inner corner of the rim but still brakes, though pad will wear a ledge over time and can stick in (not enough to slow the bike), so periodically I slide the pads out from their holders and level with a course file and put back in.

I have the new disc bike, unused yet, and ongoing saga with bike not matching advertised spec on something critical, it's on another thread.