Electric Vehicles

That's a strange and specific problem. Gas tanks don't shrink.

As another said, That is very specific, can't say I've heard of that before, but not a Prius connoisseur, However is right regardless when it comes to ICE engines (internal combustion engine). An engines efficiency goes down over time, there is a lot of mechanical movement and a lot of its components decay overtimes, bearings, piston rings, springs, valves, gaskets, just everything in the engine is not a "lifetime" part. In 20 years, if you on average drive 2 hours a day, 5 days a week at 2,000rpm, just 1 piston has done 41,600,000 strokes, crankshaft has spun 20,800,000 rotations, Camshaft has spun 10,400,000 rotations as has the intake and exhaust cam opened/closed, fuel has been injected into the engine and exploded. It comes at a cost that would honestly be similar to the lost of charge in an EV battery, however the EV battery will lose its charge at a same rate even if "neglected". An engines overall efficiency over time also correlates to the maintanace you put in. Late in a oil service can cause problems causing poor efficiency, as can using the wrong oil, or using the wrong octane/cetane rating fuel/diesel, poor engine cooling, etc, a lot of maintenance is required, and you'll still have poor efficiency after 20 years, at least with a battery, as long as you aren't constantly discharging it completely then filling it, and it is being cooled correctly, it should consistently lose its charge over time.