One of the things you've got to understand is that oil pressure doesn't protect the engine. All oil pressure does is ensure the oil goes through the passages and holes and gets to the areas where the unpressurized oil is protecting things. The oil in the bearings isn't pressurized, and it's the presense of the oil that is protecting the bearings.
So oil flow is real important, and pressure much less so. That's why engines don't die from momentary loss of oil pressure. It's the longer term loss of flow that kills things.
Sudden drops of oil pressure are typically things like pickup screen blockage, pressure relief spring breakage, cracked tube, etc. Gradual long term dropping of pressure is more indicative of engine wear, oil pump wear, etc.
As for bearings vs a rebuild, that's darn hard to tell from the outside of an engine. You can't measure bore wear and such. Only when you open the engine up can you see how things look.