I've seen a gradual, but very small, lessening of a cpu's ability to OC over time, yet it continues to keep on working just fine. I mean a long time, say a year.
Merely, its high OC limit slows a little bit, while still being clocked high, just not quite as high. Say it slows by less than 100 MHz or so. No big deal.
I've never fried a cpu, even though I've made pretty big mistakes and run them way too hot. CPUs have been very forgiving of my OC mistakes.
In the end, , at least for me it's never mattered because obsolescence happens before a cpu has degraded significantly from overclocking. YMMV.
But I only OC with air cooling, and don't know the situation for guys who use liquid or dry ice or phase change cooling and radically high OCs. I suspect they do fry CPUs sometimes.
edit: typo fix