First off, 275K is pretty good for any vehicle on the original engine. If you like this car and want to keep it, no worries, provided it tests out by a reliable inde mechanic as "worn out", you can either have your engine rebuilt or get a replacement engine. You'll be back on the road with good oil pressure in no time. If you price replacement engines for this vehicle out, I think you'll find they are remarkably economical -- compared to the sticker shock of buying a new replacement car anyway.
To address your above query, the oil pump sends pressurized oil throughout the engine via many small passages to lubricate among other things the crankshaft bearings and piston walls. There is a small oil passage that runs right down the middle of the crankshaft for example, with outlets at each of the bearings. Here's the thing: The overall oil flow rate is limited by the size of the oil delivery openings at the end of the small passages. As the engine wears over hundreds of thousands of miles driven, the bearing surfaces decay a little, so the size of the openings at the end of the oil tubes enlarge. This seems at first to be good, as more oil flows, but it is actually bad, b/c the oil pump is limited in the volume of oil it can pump per second. When a lot of oil is leaking out in certain spots, the other places the oil is supposed to go become starved of oil.
It's like if you were throwing a birthday party and trying to be efficient and blow up 5 balloons at once, so you hooked all five balloons to the same pipe, and blew into that pipe. It would work unless one of the balloons popped, in which case most of the air would go to the popped balloon, and all the other balloons would get no air at all. That's probably the situation you got with your engine.
That said, there are a lot of vehicles on the road with more than 275,000 miles on the original unrebuilt engine. How long an engine "lasts" before this happens depends on a lot of factors, including the engine design criteria back at the factor, how aggressively the engine is driven, and the routine maintenance. If you'd like to increase the chance your next car will last more than 275K before this happens, choose a model that has a history of being long-lived, drive conservatively, and follow the manufacturer's routine maintenance recommendatons to the "T".