Cleaning the slip rings probably did more for that alternator than the low miles did, since most of those reman failures start with dirty contacts or bad brushes. Denver's dry air helps too, way less corrosion on the junk parts out there than you'd get somewhere humid. Junkyard electrics are a solid bet as long as you test them before you bolt them on.

Ran into the same thing on a 172N a few years back, crack right at the lower left attach point too, and it had been signed off twice before anyone caught it. What worked for me was pulling the cowl and really cleaning the mount tubes with mineral spirits first, because grime hides those hairline cracks. Then I hit the suspect areas with dye penetrant and let it sit the full dwell time, no rushing it, and that crack showed up clear as day. Now I do the full penetrant check at every annual instead of waiting for something to look off, takes maybe 30 extra minutes but its worth it.