Back in the dark ages you filled the battery until you saw a curved rather than flat surface. Is it the same now? The battery in my 2013 Corolla was in it to my knowledge when I bought it in 2016 and I haven't changed it. I drive maybe 4K miles/year in SoCal. The battery shows NO hesitation whatever when cold starting. I'm amazed that the thing has lasted this long, but don't see a a reason to replace it.
Added some water, but didn't want to add too much -- if there is such a thing short of actual overflow. I liked the individual caps better than the 3-cap thing it has now. Overnight charging (2A, decreasing) allowed cranking at 8 and 9V.
Am I being foolish, or would it be foolish to replace this seemingly really good battery?