iPad Air, A1474, No Longer Charges- will display battery charge symbol

I can safely assume the battery is dead BUT, since the charge symbol, a battery with a very low red line, still displays, can I safely assume that the rest of the unit is still operational? Why wouldn't the unit boot up and still run when the charger is connected, in spite of the battery's condition? I'm willing to replace the battery but want to be fairly certain there isn't something else. The battery failure was a slow predictable death as I would notice less and less run time without a charger connection – until it refused to charge or boot at all. No other issues to note - the battery isn't swollen. I would give up on the unit except for the fact there is one legacy app on it my wife relies on that isn't available for newer iPads.