I wear through brake pads quickly on my electric-assist cargo bike as I'm often hauling 2 kids around San Francisco hills. I recently checked my rotors and discovered they're well past the recommended replacement thickness (Shimano recommends a minimum 1.5mm, these are reading about 1.3mm). I've replaced them, of course - not taking chances with safety, especially when it involves my kids - but the brakes were still working fine and the rotors were true. They're solid steel, SM-RT64 rotors. I can imagine that if the rotors get down to the thickness of a soda can then I'd have problems, but there's a long way to go before I'd get any where near that point. What sort of failures do disc rotors experience when they're still thick enough to be rigid? Is there some warning, or there's sudden catastrophic failure?

On a related note, the bike has TRP 4-piston brakes, and came with TRP pads, which only lasted about 700 miles. I replaced them with another pair of TRP pads and got another 700 miles out of those. I've since switched to Shimano pads - the D03s, which are resin and e-bike rated and cost the same as the TRP pads - and they're lasting 2000 miles, but perhaps at the cost of the rotors. Any opinions on whether I'm better off with the apparently-softer TRP pads to save my rotors? (The TRP pads are hard to find at a LBS, whereas the Shimano ones are readily available; haven't tried other options, and I like to support a LBS when I can.)