a couple of different thoughts
I have always believed that the bulk of moisture that rusts Lycoming cams comes from condensation of blow-by moisture inside the crankcase, rather than from exterior atmospheric moisture. This means that the state inside after 10 years depends a lot on how it was shut down the last time, and whether the interior moisture was able to breath out without condensing too much (like if it was a reasonably warm summer day when it was shut down last, or if the dip stick is removed while the engine cools down)
My soaring club bought a C-150/150 (STC for O-320-E2D) a few years ago that had been left idle for a handful of years. It got a fresh annual and went into service. I don't think we were doing oil analysis. Over a period of about 5 months of steady operation (probably 150 hrs) the oil consumption steadily increased and the power output steadily declined, although it otherwise seemed to run fine. It got to where it was using a quart of oil every two hours or so, and could only turn the prop at 2350 rpm at Vy, so we finally sent it in for overhaul. It was basically a core. I didn't hear back specifically, but my assumption is that the cam wore out so there was little valve lift, and the rings wore out, probably from chewing on cam metal dispersed into the oil.
A subtext of the above story is that the engine was "safe" to operate for a long time, until the safety limit was unacceptable power output, not because of immanent mechanical failure.
One thing about the frequent advise to pull a cylinder and have a look -- it seems fairly straightforward to pull a cylinder until you consider how much work is involved in removing the cooling baffles, all nicely sealed with orange RTV, and all the other accessories that have to come off first, at the very least includes the exhaust pipes, cylinder and exhaust temperature probes, induction tube, oil drain tube, and all of the nut-launchers (Adel clamps) that are attached to all those things to support them or other items. By the time all that stuff is off, it starts to seem pretty easy to just pull the whole engine.
Finally -- one poster mentioned something about a 12-year TBO? I never heard of that before. I don't think I've ever seen that in any of my Lycoming documents??? I ran a IO-320 in a Citabria for 3200 hrs before a field overhaul. So I don't know that I'd care much about a 12-year limit.