In that post about the oil plug, the machine shop had removed the plug and not replaced it. It is highly unlikely that your plug has come out. You are probably dealing with wear on the wear on the crankshaft and cam bearings and wear in the oil pump. A new high volume oil pump would probably help the oil pressure, but an engine rebuild would be the only cure. You could replace the crankshaft bearings and rear seal while you had the oil pan off, to replace the oil pump, and have a mini rebuild. The engine should last a long time with low oil pressure if you take care of it.