Imagine that some cars allow the gas tank to be changed by the user. But this one doesn't.

Imagine the company that makes the car never mentions the shrinkage or the gradual reduction in top speed, and has no program or system that allows you to fix this - beyond buying a newer car.

To try to salvage your analogy, I'll put forth an anecdote. Last year I restored my deceased father's 1988 Bronco II. One of the issues it had was low fuel pressure, which I narrowed down to the in-tank fuel pump (the Bronco was over-engineered and had two pumps, one in the tank and one on the frame). It wasn't able to push fuel fast enough for the engine to run at top performance. Sound familiar? So I had two choices, replace it myself or take it to a shop, kind of like how you can attempt to replace your phone's battery if you know what you're doing, or take it to a repair shop and let a pro do it for a fee. I'm a competent shadetree mechanic, so I did it myself and saved a few hundred dollars, and it is back to normal operation again.